Tighten requirements generation and testing

[kurtmckee]

This PR introduces these changes:

• test-mindeps requirements are only generated for Python 3.7
• docs and typing requirements are only generated for Python 3.11
• CI explicitly uses Python 3.11, not any Python 3 version
• typing-mindeps no longer exists and will not be tested
• The Sphinx version restriction is lifted because furo theme was patched in the last 20 hours

This should close most (if not all) Dependabot PRs that were opened against the testing requirements files.

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📚 Documentation preview 📚: https://globus-sdk-python--819.org.readthedocs.build/en/819/

[sirosen]

I checked our other "3.x" builds to see if anything else looks like it will be impacted, and I think we'll be fine. We can always tweak CI things as needed, but this seems like the right spec in terms of our requirements x versions x builds. So if there's any change needed, it will be minor.

Also, for the public record: we're pretty sure that typing-mindeps isn't an effective test. I'm under the impression that type checkers may bundle/vendor typing-extensions rather than using the installed version.