Drop generated source from source distribution

[methane]

Thanks to cibuildwheel, we can provide wheels for most common platforms.
Dropping C++ source from source distribution will make build process simple.

[jfolz]

IMO it makes sense to have generated code in the source distribution. Other than removing Cython as a build dependency on the user's end, we can also be sure what code was compiled without worrying about bugs in different versions of Cython.

[methane]

We can pin the version of cython to generate source by pyproject.toml.

https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/#build-time-dependencies