Migrate to "pure Python" Cython mode where applicable

[vytas7]

As of Cython 3.0+, it is possible to write most of the Cython code as pure Python, 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.

[CaselIT]

It's also possible to import a compiled pure python cython module without using the compiled code, by doing a bit of shenanigans with the import machinery. This is useful for tests and benchmarks:

_M = TypeVar("_M", bound=ModuleType)
def load_uncompiled_module(module: _M) -> _M:
    """Load the non-compied version of a module that is also
    compiled with cython.
    """
    full_name = module.__name__
    assert module.__spec__
    parent_name = module.__spec__.parent
    assert parent_name
    parent_module = sys.modules[parent_name]
    assert parent_module.__spec__
    package_path = parent_module.__spec__.origin
    assert package_path and package_path.endswith("__init__.py")
    name = full_name.split(".")[-1]
    module_path = package_path.replace("__init__.py", f"{name}.py")
    py_spec = importlib.util.spec_from_file_location(full_name, module_path)
    assert py_spec
    py_module = importlib.util.module_from_spec(py_spec)
    assert py_spec.loader
    py_spec.loader.exec_module(py_module)
    return cast(_M, py_module)

[vytas7]

One unpleasant gotcha so far is that cython needs to be imported directly in a module that is being cythonized, importing via another module/compat loader doesn't work for some reason. Probably it is AST-parsed by Cython instead of using the normal import machinery.

[vytas7]

Another glitch was that some aliases were missing, but this will be fixed in the next Cython release: cython/cython#5934.