style: Add pep8-naming plugin to flake8
style: Add pep8-naming plugin to flake8

Enforce PEP8 naming conventions for things like variables, methods,
classes, etc.

See the table here:

https://pypi.org/project/pep8-naming/

...for error codes emitted, in case some should be added as
exceptions in the flake8 configuration files.

krafczyk/spack,mfherbst/spack,krafczyk/spack,krafczyk/spack,LLNL/spack,mfherbst/spack,LLNL/spack,mfherbst/spack,iulian787/spack,iulian787/spack,krafczyk/spack,iulian787/spack,LLNL/spack,mfherbst/spack,iulian787/spack,iulian787/spack,LLNL/spack,mfherbst/spack,krafczyk/spack,LLNL/spack