Specifying a floor, but not a ceiling, for core dependencies should help
prevent dependency resolution conflicts. Dependencies that affect
functional test output are pinned more tightly.

Pinning dependencies to specific versions creates conflicts when other
packages require more recent versions. We can do our part by allowing
for a wider range of dependency versions, specifying only the oldest
version that will still work. Meanwhile, we ensure that test
environments use a specific pinned dependency in order to match the
expected functional test output.

Otherwise, installation conflicts can result when plugins/themes require
more recent Pygments versions.

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

Also, stop pinning pytest version, since older version combined with
pytest-xdist 2.0 caused CI test errors.

- 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

Uses pytest-xdist to run tests on two cores simultaneously, speeding up
test runs by nearly 40%.