PEP: pandas/core round 3 (generic, groupby, index)

yapf with manual modifications for error strings and bad yapf
formatting.

Author: rockg <grant.roch@gmail.com>

Closes #12062 from rockg/pep8-round3 and squashes the following commits:

74c9dff [rockg] PEP: pandas/core round 3 (generic, index)

Author: rockg