MAINT: remove requirements-dev.txt; expand optional dependencies in pyproject.toml

[mwtoews]

This PR has a few related aims:

• Remove requirements-dev.txt
• Expand the optional dependencies (in pyproject.toml) for "test" and "docs"

[coveralls]

Pull Request Test Coverage Report for Build 3459652741

• 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 84.815%

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Totals
Change from base Build 3459418425:	0.0%
Covered Lines:	2195
Relevant Lines:	2588

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💛 - Coveralls

[jorisvandenbossche]

There was also already #1564, but I think updating the existing "test" and "docs" extras as you do here is better.

[mwtoews]

I thought it was familiar! I don't think "cython>=0.29.32" is important, as typically this is done in an isolated environment were the latest versions are retrieved from PyPI.