Upgrade CI tools

[Gallaecio]

Includes:

• Downgrading flake8 and isort for Python 3.7 support.
• Replacing tox environments for bandit, flake8, and black with one for pre-commit.
• Code changes due to upgrading black. Most notably, removing empty lines after class/def lines.

[codecov]

Codecov Report

Merging #5823 (32a01e3) into master (c34ca4a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5823   +/-   ##
=======================================
  Coverage   88.94%   88.94%           
=======================================
  Files         162      162           
  Lines       11002    11002           
  Branches     1798     1798           
=======================================
  Hits         9786     9786           
  Misses        937      937           
  Partials      279      279           

Impacted Files	Coverage Δ
scrapy/commands/__init__.py	74.71% <ø> (ø)
scrapy/commands/bench.py	100.00% <ø> (ø)
scrapy/commands/crawl.py	60.00% <ø> (ø)
scrapy/commands/edit.py	51.85% <ø> (ø)
scrapy/commands/fetch.py	89.13% <ø> (ø)
scrapy/commands/genspider.py	87.38% <ø> (ø)
scrapy/commands/list.py	77.77% <ø> (ø)
scrapy/commands/runspider.py	93.47% <ø> (ø)
scrapy/commands/settings.py	71.87% <ø> (ø)
scrapy/commands/shell.py	92.85% <ø> (ø)
... and 32 more

[Gallaecio]

Note: Whichever gets merged first, this or #5816, requires the other pull request to be updated accordingly to have black versions match.

[kmike]

Should we include pre-commit to github actions "checks" workflow? Any other changes needed to the workflow?

[Gallaecio]

We already run pre-commit in GitHub (without tox).

Since the pre-commit configuration freezes versions at the pre-commit level, and all other checks use tox, I don’t think we need anything else.

[kmike]

Thanks @Gallaecio!