Replace dev-requirements.txt with a [dev] extra

[SnoopJ]

Description

This PR removes dev-requirements.txt and moves the optional development dependencies it defines into a new [dev] extra. Closes #2363

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

I will happily merge this once squashed. Maybe the final commit message is something like install: use [dev] extra instead of dev-requirements.txt?

(We like our easily scannable module/component prefixes, if you couldn't tell. 😛)

[SnoopJ]

I will happily merge this once squashed. Maybe the final commit message is something like install: use [dev] extra instead of dev-requirements.txt?

(We like our easily scannable module/component prefixes, if you couldn't tell. stuck_out_tongue)

Yea, I just didn't know which one made the most sense here, will use install in the squash.

Can you drop the best reference on the flake8 issue this uncovered?

[dgw]

Can you drop the best reference on the flake8 issue this uncovered?

Uh, that's probably PyCQA/flake8#1004 though there are numerous threads from consumers trying to solve the incompatibility later (post-release) on both flake8's and importlib_metadata's repos.

[dgw]

Ultimately reverted in #2392, due to workflow issues we can't solve with pip. The "eager" upgrade strategy doesn't work the way we thought, so it became difficult to keep dev-deps up to date.