Migrate to "pure Python" Cython mode where applicable

As of Cython 3.0+, it is possible to write most of the [Cython code as pure Python](https://cython.readthedocs.io/en/latest/src/tutorial/pure.html), leveraging decorators and type annotations to convey the desired C compilation behaviour.

This would require `cython` at runtime, but we can shim only the annotations in order to avoid a hard dependency. @CaselIT has already implemented a prototype in this vein for SQLAlchemy.

We might want to keep `.pyx` for some tight-knit C character loops used for parsing parameters, but, for instance, multipart stream reader is mostly about typing with `Py_ssize_t`, and calling internal helpers in C, so we should be able to easily rewrite it in such mixed mode.

The main benefit is obviously that we don't need to maintain two different codebases for the same thing. Also, even if we keep dual implementations, we would be able to easily test implementation in pure Python unit tests, tracking coverage.

Start by inventorying the existing .pyx modules and reviewing the multipart stream reader, while identifying the tight parsing loops that should remain in .pyx. Consult Cython's pure-Python mode documentation and the SQLAlchemy prototype for the annotation shim approach. Done means applicable Cython code has one maintainable implementation and can be exercised by pure-Python unit tests.

Written by the indexing model from the issue text.