Tests compatibility with packaging 26.0

[frenzymadness]

In packaging release 26.0, they've changed the logic for handling prereleases. See pypa/packaging#897

Because of that change, some tests in this project fail like this:

=================================== FAILURES ===================================
____________________________ test_resolver[cheroot] ____________________________

provider = <test_resolvers_python.PythonInputProvider object at 0x7fd8ed1d8f30>
reporter = <conftest.TestReporter object at 0x7fd8ebc00050>

    def test_resolver(provider, reporter):
        resolver = Resolver(provider, reporter)
    
        if provider.expected_confliction:
            with pytest.raises(ResolutionImpossible) as ctx:
                result = resolver.resolve(provider.root_requirements)
                print(_format_resolution(result))  # Provide some debugging hints.
            assert _format_confliction(ctx.value) == provider.expected_confliction
        else:
            resolution = resolver.resolve(provider.root_requirements)
>           assert _format_resolution(resolution) == provider.expected_resolution
E           AssertionError: assert {'atomicwrite...8.6.0')>, ...} == {'atomicwrite...8.6.0')>, ...}
E             
E             Omitting 13 identical items, use -vv to show
E             Differing items:
E             {'pluggy': <Version('1.0.0.dev0')>} != {'pluggy': <Version('0.13.1')>}
E             {'pyparsing': <Version('3.0.0b1')>} != {'pyparsing': <Version('2.4.7')>}
E             
E             Full diff:...
E             
E             ...Full output truncated (23 lines hidden), use '-vv' to show

tests/functional/python/test_resolvers_python.py:176: AssertionError
----------------------------- Captured stdout call -----------------------------
Pin  Candidate(name='coverage', version=<Version('5.3')>, extras=set())
 Pin  Candidate(name='pytest-mock', version=<Version('3.3.1')>, extras=set())
  Pin  Candidate(name='pytest-sugar', version=<Version('0.9.4')>, extras=set())
  Reject Candidate(name='pytest-testmon', version=<Version('0.9.19')>, extras=set())
 Reject Candidate(name='pytest-testmon', version=<Version('0.9.18')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.16')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.15')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.14')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.13')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.12')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.11')>, extras=set())
Pin  Candidate(name='pytest-sugar', version=<Version('0.9.3')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.19')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.18')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.16')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.15')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.14')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.13')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.12')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.11')>, extras=set())
Pin  Candidate(name='pytest-mock', version=<Version('3.3.0')>, extras=set())
Pin  Candidate(name='pytest-sugar', version=<Version('0.9.4')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.19')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.18')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.16')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.15')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.14')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.13')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.12')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.11')>, extras=set())
Pin  Candidate(name='pytest-sugar', version=<Version('0.9.3')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.19')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.18')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.16')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.15')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.14')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.13')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.12')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.11')>, extras=set())
Pin  Candidate(name='pytest-mock', version=<Version('3.2.0')>, extras=set())
Pin  Candidate(name='pytest-sugar', version=<Version('0.9.4')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.19')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.18')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.16')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.15')>, extras=set())
Reject Candidate(name='pytest-testmon', version=<Version('0.9.14')>, extras=set())
Pin  Candidate(name='pytest-testmon', version=<Version('0.9.13')>, extras=set())
Pin  Candidate(name='packaging', version=<Version('20.4')>, extras=set())
Pin  Candidate(name='pyparsing', version=<Version('3.0.0b1')>, extras=set())
Pin  Candidate(name='pytest', version=<Version('3.10.1')>, extras=set())
Pin  Candidate(name='atomicwrites', version=<Version('1.4.0')>, extras=set())
Pin  Candidate(name='attrs', version=<Version('20.3.0')>, extras=set())
Pin  Candidate(name='more-itertools', version=<Version('8.6.0')>, extras=set())
Pin  Candidate(name='pluggy', version=<Version('1.0.0.dev0')>, extras=set())
Pin  Candidate(name='py', version=<Version('1.9.0')>, extras=set())
Pin  Candidate(name='setuptools', version=<Version('50.3.2')>, extras=set())
Pin  Candidate(name='six', version=<Version('1.15.0')>, extras=set())
Pin  Candidate(name='termcolor', version=<Version('1.1.0')>, extras=set())

[notatallshaw]

I can take a look this week and fix.

[frenzymadness]

@notatallshaw do you need any assistance with this?

[notatallshaw]

Thanks for the ping, I totally forgot about this as I was busy with the pip 26.0 release, I have a fix: #201