STY: pep8 and pep257 audit.

Audit numpy/lib/arraypad.py for pep8 and pep257 compliance.
Also fix a few minor docstring corrections converting ] into ) or
vice versa.
Merge pull request #3310 from JDWarner/cleanup_arraypad

Cleanup arraypad.py (PEP8, PEP257) and correct docstrings
BUG: fix potentially infinite recursion in VOID_nonzero. Fixes #3312.
TST: add a test for issue #3312
Merge pull request #3313 from ecatmur/master

BUG: fix potentially infinite recursion in VOID_nonzero. Fixes #3312.