Add Python < 3.6 support to tests

The package should work with Python 2.7 or any version of Python, since
it's a data-only package, but some of the testing infrastructure no
longer supports Python 2.7.

For the moment, we'll make sure it works with 2.7, though we will likely
not continue to test on 2.7 indefinitely.

Author: pganssle

tests/test_contents.py

@@ -1,9 +1,29 @@
+import sys
+
+import pytest
+
 try:
     from importlib import resources
 except ImportError:
     import importlib_resources as resources
 
-import pytest
+
+if sys.version_info < (3, 6):
+    # pytest-subtests does not support Python < 3.6, but having the tests
+    # separated into clean subtests is nice but not required, so we will create
+    # a stub that does nothing but at least doesn't fail for lack of a fixture.
+    import contextlib
+
+    class _SubTestStub:
+        @contextlib.contextmanager
+        def test(self, **kwargs):
+            yield
+
+    _sub_test_stub = _SubTestStub()
+
+    @pytest.fixture
+    def subtests():
+        yield _sub_test_stub
 
 
 def get_magic(zone_name):

tox.ini

@@ -7,7 +7,7 @@ skip_missing_interpreters = true
 description = Test that the tzdata contents are accessible
 deps =
     pytest
-    pytest-subtests
+    pytest-subtests; python_version>='3.6'
     importlib_resources; python_version<'3.7'
 commands =
     pytest {toxinidir} {posargs}