PTY is not supported on Windows.

Someday the entire Pelican code base will perhaps have Black and isort
formatting applied to it, but it doesn't make sense for the linter to
check that until it's actually done. This changes the `lint` Invoke task
to run Flake8 on changed lines only, which should bring it into line
with both the current Pre-commit and CI configurations.

Also, the docs erroneously stated that `invoke tests` would also check
for code style compliance, which has herein been fixed.

psutil does a much better job at accurately counting CPU cores,
of which pytest-xdist takes advantage for the purposes of running
tests concurrently.

- pytest: verbose isn't needed but show skipped tests
- mock is not needed anymore. it is part of stdlib
- Unpin Sphinx and fix warning about lexer
- Mirror tox command for doc build in tasks.py