feat: Stage 3-4 of nox implementation - adding auto-format targets

[mf2199]

As suggested in #466 , this represents item 4 of the recommended list aimed to implement nox testing automation in multiple steps.

Change list:

1. Sessions lint and blacken added to the Nox configuration;
2. Relevant folders added as targets for blacken routines;
3. Nox executed using the updated configuration, resulting in multiple auto-format changes.

As multiple Python versions already present in unit session, no separate PR for item 3 is necessary at this point.

The lint session has conflicting definitions among flake8 and blacken (the latter follows PEP8). This is a known error, discussed, for example, here and here. The fix involves adding exceptions to the flake8 configuration file.

Session log:

nox > Running session lint
nox > Creating virtual environment (virtualenv) using python.exe in .nox\lint
nox > pip install flake8 black==19.10b0
nox > black --check django_spanner docs spanner_dbapi tests noxfile.py setup.py
All done! ✨ � ✨
35 files would be left unchanged.
nox > flake8 django_spanner spanner_dbapi tests
nox > Session lint was successful.
nox > Running session blacken
nox > Creating virtual environment (virtualenv) using python.exe in .nox\blacken
nox > pip install black==19.10b0
nox > black django_spanner docs spanner_dbapi tests noxfile.py setup.py
All done! ✨ � ✨
35 files left unchanged.
nox > Session blacken was successful.
nox > Running session lint_setup_py
nox > Creating virtual environment (virtualenv) using python.exe in .nox\lint_setup_py
nox > pip install docutils pygments
nox > python setup.py check --restructuredtext --strict
running check
nox > Session lint_setup_py was successful.
nox > Ran multiple sessions:
nox > * lint: success
nox > * blacken: success
nox > * lint_setup_py: success

Towards #474.

[c24t]

LGTM. The only issue I see is black/isort incompatibility, but that's only an issue for people (like me) running isort locally since we don't call it from nox. Let me try to reconcile this with #469.