AttributeError when finder has no __module__

As reported in #91 (closed):

[The fix for #91 (closed)] broke compatibility with PyOxidizer whose finder is not attached to a module:

Traceback (most recent call last):
  [...]
  File "importlib_metadata", line 471, in <module>
  File "importlib_metadata._compat", line 62, in install
  File "importlib_metadata._compat", line 79, in disable_stdlib_finder
  File "importlib_metadata._compat", line 76, in matches
AttributeError: 'PyOxidizerFinder' object has no attribute '__module__'