tox flake8-isort exclude env and .venv

[n2ygk]

Description of the Change

Updates setup.cfg to ignore the .venv/ or env/ directories as was already the case in .gitconfig.
This allows having a local virtualenv that tox -e flake8 doesn't scan.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #645 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #645   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files          56       56           
  Lines        2870     2870           
=======================================
  Hits         2760     2760           
  Misses        110      110

Impacted Files	Coverage Δ
rest_framework_json_api/django_filters/backends.py	100% <ø> (ø)	⬆️
example/tests/test_views.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8bb123c...90230cf. Read the comment docs.

[codecov]

Codecov Report

Merging #645 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #645   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files          56       56           
  Lines        2870     2870           
=======================================
  Hits         2760     2760           
  Misses        110      110

Impacted Files	Coverage Δ
rest_framework_json_api/django_filters/backends.py	100% <ø> (ø)	⬆️
example/tests/test_views.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8bb123c...90230cf. Read the comment docs.

[sliverc]

LGTM