Fixed skipped test

In `test_pep257.py`, the first test was skipped with `pytest.skip`, but unfortunately due to a "bug" in their API, it caused all of the tests in that module to be skipped as well (I don't know if this was on purpose - @keleshev?)
For details on this behavior of pytest, check out https://bitbucket.org/hpk42/pytest/issue/607/pytestskip-will-skip-entire-files-when
Since the tests didn't run for quite some time, most of them were obsolete and tested functions and API that didn't exist anymore. Therefore, I deleted most of them and corrected a few that were still relevant.
Also, there was an error with the testing the conformance of `test_pep257` with the `pep257.py` script which raised an `AllError`. It was caused by a problem with the parsing of `__all__`, which was also fixed.