Macro maybe_define_fastcall_function_with_keywords 

#[doc(hidden)]
macro_rules! maybe_define_fastcall_function_with_keywords {
    ($name:ident, $py:ident, $slf:ident, $args:ident, $nargs:ident, $kwargs:ident, $body:block) => { ... };
}

Available on Py_3_10 or non-Py_LIMITED_API only.

Abstraction around fastcall calling convention, which is only available in Python 3.10 and up, can inline this directly into the proc macro when Python 3.10 support dropped