Non-types dependencies: whitelist arrow, click, Flask, Werkzeug

[AlexWaygood]

This PR adds four new items to the "non-types dependencies whitelist", in preparation for non-types dependencies being allowed in typeshed: arrow, click, Flask, and Werkzeug.

arrow

Typeshed has had a PR open to add stubs for python-datemath for a long time now: python/typeshed#5765. The arrow package is fundamental to python-datemath, so the PR realistically can't be merged without having arrow as a dependency. At runtime, arrow only depends on python-dateutils and, on Python <3.8, typing_extensions: https://github.com/arrow-py/arrow/blob/74a759b88447b6ecd9fd5de610f272c8fb6130a2/setup.py#L25-L28. It is actively maintained, and has 8.2k stars on GitHub. arrow is the sole dependency of python-datemath at runtime: https://github.com/nickmaccarthy/python-datemath/blob/6d047b3cc4e357d83b50e7433c812994e21c3ffb/setup.py#L86.

click

Typeshed has a PR open to add stubs for the click-default-group: python/typeshed#9304. Similar to the PR adding stubs for python-datemath, there's not much point merging the PR unless it can declare a dependency on click; click is just too fundamental to the package. click is obviously a hugely popular CLI framework for Python. Its only dependencies at runtime are colorama (if you're on Windows) and importlib-metadata (if you're on Python <3.8): https://github.com/pallets/click/blob/9595a190d79e80945f6827a79f12937a8212f307/setup.py#L5-L8. click is the sole dependency of click-default-group at runtime: https://github.com/click-contrib/click-default-group/blob/b671ae5325d186fe5ea7abb584f15852a1e931aa/setup.py#L52.

Flask and Werkzeug

Typeshed has stubs for several Flask plugins. Being able to declare a dependency on Flask would be useful for typeshed's stubs for Flask-Cors and Flask-Migrate in particular. Flask is declared as a dependency of both Flask-Cors and Flask-Migrate at runtime. Flask is obviously a hugely popular web framework for Python, that is actively maintained. It has a few more dependencies at runtime than the others on this list, but still not very many: https://github.com/pallets/flask/blob/836866dc19218832cf02f8b04911060ac92bfc0b/setup.py#L6-L12.

Typeshed's stubs for Flask-Cors would also benefit from being able to declare a dependency on Werkzeug: https://github.com/python/typeshed/blob/41de5317b5008719c899b0ca61f9fd37b774cade/stubs/Flask-Cors/flask_cors/core.pyi#L12. The benefit to this one would be fairly small, but it also seems silly to disallow declaring a dependency on Werkzeug, since Werkzeug is already a dependency of Flask at runtime, and is just as actively maintained as Flask.