Upgrade black and enforce minimum Tox version

[jadchaar]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [ ] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [ ] 📚 Updated documentation for changed code.
• [x] ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Black now has an opinion about docstrings and removes trailing and leading whitespace in docstrings. Also, I am enforcing a minimum tox version since the keyword whitelist_exteranls has been replaced by allowlist_externals.

[codecov[bot]]

Codecov Report

Merging #846 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #846   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines         1785      1785           
  Branches       307       307           
=========================================
  Hits          1785      1785           

Impacted Files	Coverage Δ
arrow/api.py	100.00% <ø> (ø)
arrow/arrow.py	100.00% <ø> (ø)
arrow/factory.py	100.00% <ø> (ø)
arrow/locales.py	100.00% <ø> (ø)
arrow/parser.py	100.00% <ø> (ø)

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