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.

.travis.yml

@@ -146,6 +146,7 @@ install:
   - pip install --upgrade setuptools
   - pip install --upgrade codecov
   - pip install --upgrade flake8
+  - pip install --upgrade pep8-naming
   - if [[ "$TEST_SUITE" == "doc" ]]; then pip install --upgrade -r lib/spack/docs/requirements.txt; fi
 
 before_script: