bpo-32991: Restore expectation that inspect.getfile raises TypeError on namespace...

bpo-32991: Restore expectation that inspect.getfile raises TypeError on namespace package (GH-5980) (GH-5997)

* bpo-32991: Add test capturing expectation.

DocTestFinder.find should return an empty list for doctests in a namespace package.

* bpo-32991: Restore expectation that inspect.getfile on a namespace package raises TypeError.
(cherry picked from commit b9650a04a81355c8a7dcd0464c28febfb4bfc0a9)

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>