Docs: pin Cython=0.29, geos=3.11, add pip for ReadTheDocs

[mwtoews]

Since Cython 3.0 was released a few weeks ago, the ReadTheDocs builds have been failing:

$ python ./setup.py install --force
...
Error compiling Cython file:
------------------------------------------------------------
...
        self.last_error = <char *> malloc((1025) * sizeof(char))
        self.last_error[0] = 0
        self.last_warning = <char *> malloc((1025) * sizeof(char))
        self.last_warning[0] = 0
        GEOSContext_setErrorMessageHandler_r(
            self.handle, &geos_message_handler, self.last_error
                         ^
------------------------------------------------------------

shapely/_geos.pyx:30:25: Cannot assign type 'void (*)(const char *, void *) except *' to 'GEOSMessageHandler_r'

While this is a curious bug to fix for Cython 3.x, the short-term solution is to pin the docs/environment.yml for now. (This error doesn't occur in the main build, since pyproject.toml has "Cython~=0.29" in build-system.requires)

This also upgrades geos to 3.11 which is the same version used for the binary wheels. Furthermore, add pip for ReadTheDocs (which auto-adds a few pip deps).

Also, the RTD option described in #1857 is now toggled, so we should see the build in this PR.

[coveralls]

Pull Request Test Coverage Report for Build 5708402245

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.

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• 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 increased (+0.3%) to 87.759%

---

Totals
Change from base Build 5630223519:	0.3%
Covered Lines:	2545
Relevant Lines:	2900

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💛 - Coveralls

[mwtoews]

Also, the RTD option described in #1857 is now toggled, so we should see the build in this PR.

Not yet. I don't have sufficient permissions to enable RTD to send GitHub build status report back to this repo.

[mwtoews]

Since maint-2.0 was failing too, this is backported with 75aebf6