The output of python-library had a number of lint, quality, and test failures. This commit 1) fixes all of those failures, and 2) ensures that we have test coverage of those quality checks so that we don't regress in the future.

Fixes to quality checks and unit tests:

• Don't try to pylint manage.py in base template, since not all of them have one, and this causes a quality check failure for python-library. Simply removing the explicit reference is fine, since it's already covered by *.py.
• Add a placeholder test so pytest will succeed for python-library. (It fails if there are no tests.) This placeholder test is marked to be skipped, which will hopefully stand out a bit in the resulting repo's test runs.
• Move doc theme overrides CSS to base template (deleting from layered cookiecutters). There's a line in python-template's doc config that expects a _static dir (html_static_path = ['_static']) but there's no dir in the base, so docs build breaks for python-library (which did not have a copy of the CSS in a _static dir). Moving the CSS to the base fixes this so all cookiecutters can use it.
• Remove logo_only from base docs config, as it was breaking the docs build for python-library.
  • sphinx-book-theme 1.x apparently has an undocumented breaking change in Remove JQuery and update versions executablebooks/sphinx-book-theme#668 -- logo_only is no longer supported, and instead there is a logo option that I haven't looked into. I'm not sure what we want, but I'd rather unbreak and move on so that we at least don't have a broken docs build.
• Prevent duplicate target error in various READMEs.
  • The "Getting Help" section header creates an implicit duplicate target that conflicts with the _Getting Help: reference. Rather than doing something more complicated, just inline the link. (This required using a double-underscore link to make it anonymous, otherwise you still get an implicit target!)
  • But also this "getting started" text was copied into the cookiecutter readmes, which are instructions for using the cookiecutters, not used in the template output. This duplicates the root readme in the repo, so I removed that text from those readmes. (Same for in the repo's own readme.)

Other improvements:

• Simplify python-library's test_upgrade and test_quality unit tests to just be a single call to make upgrade requirements test-all. This allows us to exercise the cookiercutter's Makefile and run all of its quality checks without reproducing the full list of checks in test code. (I haven't tackled the other cookiecutters yet, since some of them actually have test failures if you try to use this approach, or don't yet have a make test-all or equivalent.)
• Include the docs build and validation in python-library's unit tests.
• Update django-app's placeholder test to match the skipped one in cookiecutter-python-library.
• Remove deprecated --recursive arg from isort calls. (Redundant in isort 5+ and causes a warning.) Not related to python-library cc tests, but easy fix.

Merge checklist:
Check off if complete or not applicable:

• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, deadlines, tickets