Fix python_requires to unbreak installed package metadata

The '>=3.6.*' entry in python_requires is invalid and results in broken
metadata being installed.  This in turn causes distlib to break.
To reproduce:

    $ pip install distlib symengine
    $ python -c "import distlib.database; \
      distlib.database.DistributionPath().get_distribution('symengine')"
    Traceback (most recent call last):
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/metadata.py", line 677, in __init__
        self._data = json.loads(data)
      File "/usr/lib/python3.9/json/__init__.py", line 346, in loads
        return _default_decoder.decode(s)
      File "/usr/lib/python3.9/json/decoder.py", line 337, in decode
        obj, end = self.raw_decode(s, idx=_w(s, 0).end())
      File "/usr/lib/python3.9/json/decoder.py", line 355, in raw_decode
        raise JSONDecodeError("Expecting value", s, err.value) from None
    json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/database.py", line 240, in get_distribution
        self._generate_cache()
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/database.py", line 167, in _generate_cache
        for dist in self._yield_distributions():
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/database.py", line 148, in _yield_distributions
        metadata = Metadata(fileobj=stream, scheme='legacy')
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/metadata.py", line 686, in __init__
        self._legacy = LegacyMetadata(fileobj=StringIO(data),
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/metadata.py", line 261, in __init__
        self.read_file(fileobj)
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/metadata.py", line 359, in read_file
        self.set(field, value)
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/metadata.py", line 459, in set
        if not scheme.is_valid_constraint_list(value):
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/version.py", line 716, in is_valid_constraint_list
        return self.is_valid_matcher('dummy_name (%s)' % s)
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/version.py", line 703, in is_valid_matcher
        self.matcher(s)
      File "/tmp/venv3/lib/python3.9/site-packages/distlib/version.py", line 115, in __init__
        raise ValueError('\'.*\' not allowed for '
    ValueError: '.*' not allowed for '>=' constraints

Author: mgorny

setup.py

@@ -222,7 +222,7 @@ def finalize_options(self):
       author_email="[email protected]",
       license="MIT",
       url="https://github.com/symengine/symengine.py",
-      python_requires='>=3.6.*,<4',
+      python_requires='>=3.6,<4',
       zip_safe=False,
       cmdclass = cmdclass,
       classifiers=[