Split `extremal-python-dependencies` out into its own subpackage

[garrison]

With this, extremal-python-dependencies can be installed from another package's CI script. Hopefully this will be a step toward allowing us to avoid some code duplication across repositories.

Remaining action items

• [ ] add a README
• [x] provide minimum versions for toml and typer

[coveralls]

Pull Request Test Coverage Report for Build 10086659149

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 9847341625:	0.0%
Covered Lines:	2410
Relevant Lines:	2410

---

💛 - Coveralls