Update tox to mirror CI and support running tests in parallel

[rmartin16]

Changes

• Updated the default behavior of tox to better reflect the checks that are run in CI
  • This continues to include testing all Python versions
  • Additionally, coverage, pre-commit, docs-lint, and towncrier-check all run now
  • The best experience will be achieved by running tox p
• The tests can be run in parallel now
  • Using tox -e py-fast will skip coverage and run tests using as many cores as the machine has
• Producing coverage reports was broken out in to tox -e coverage
  • To include the HTML report, run tox -e coverage-html
  • To fail if the coverage is too low, run tox -e coverage-fail

Notes

• This is a superior PR to Run tox tests in parallel #1247 since it separates the complexity of running tests in parallel from coverage
• In Add support for accurate local coverage reports #1244, I'm planning to round this all out by allowing coverage to be accurately calculated locally.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

After a lot of rabbit holes, I think this brings tox to a really good position now.

[freakboy3742]

Can't find a fault here - an awesome collection of speedups, consolidations and UX improvements. Nice work.