Add some tests for check_requires_python().

tests/unit/test_packaging.py

@@ -0,0 +1,22 @@
+import pytest
+from pip._vendor.packaging import specifiers
+
+from pip._internal.utils.packaging import check_requires_python
+
+
+@pytest.mark.parametrize('version_info, requires_python, expected', [
+    ((3, 6, 5), '== 3.6.4', False),
+    ((3, 6, 5), '== 3.6.5', True),
+    ((3, 6, 5), None, True),
+])
+def test_check_requires_python(version_info, requires_python, expected):
+    actual = check_requires_python(requires_python, version_info)
+    assert actual == expected
+
+
+def test_check_requires_python__invalid():
+    """
+    Test an invalid Requires-Python value.
+    """
+    with pytest.raises(specifiers.InvalidSpecifier):
+        check_requires_python('invalid', (3, 6, 5))