Tests failing on Python 3.6

[jaraco]

In 473465e, I updated the code to use a new interface for entry points. When I did, I tested in on Python 3.9. But when I pushed it for release, the tests were failing on Python 3.6 with:

>       for ep in metadata.entry_points(group='keyring.backends'):
E       TypeError: 'group' is an invalid keyword argument for this function

In this test, metadata.entry_points is monkey-patched to importlib_metadata.EntryPoints, a tuple subclass.

Investigating further, I learned that on Python 3.7+, tuple subclasses accept and ignore arbitrary keyword arguments, but on Python 3.6, those arguments are rejected.

$ python3.8 -c "type('Foo', (tuple,), {})(name='xyz')"
$ python3.9 -c "type('Foo', (tuple,), {})(name='xyz')"
$ python3.7 -c "type('Foo', (tuple,), {})(name='xyz')"
$ python3.6 -c "type('Foo', (tuple,), {})(name='xyz')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: 'name' is an invalid keyword argument for this function