Using FASTCALL for methods accepting **kwargs can't skip creating dict in most cases.  Because they accepts dict.

Even worth, when calling it like `d.update(**config)` (yes, it's abuse of
**, but it can be happen in some C methods), FASTCALL may unpack the passed
dict, and pack it again.

So, when considering METH_FASTCALL, supporting **kwargs is lowest priority.
(Off course, supporting it by AC with METH_KEYWORDS is nice to have)