Remove rust-cpython support

[messense]

Discussed in #2031

^{Originally posted by henryiii April 9, 2024}
Currently, there's a list of options when starting a new project that includes rust-cpython. Upon inspection, it turns out that rust-cpython was declared abandoned 7 months ago (2+ years since last meaningful commit, not counting this notice) and doesn't support Python 3.12+. There's a note in the readme that projects should move to PyO3.

Shouldn't this be removed from the new project options, to simplify the user choice? Maybe the auto detection code could be removed too, but at least taking it out of the templates seems reasonable at this point. I have actually had someone ask me whether they should pick pyo3 or rust-python a while back because they saw it in this list.

Phase out plan

• Remove from new/init templates: Remove rust-cpython from project init/new template #2034
• Remove from readme/documentation: Remove rust-cpython support from documentation #2057
• Remove build support: Remove support for rust-cpython #2044