Issue #29259: Write fast path in _PyCFunction_FastCallKeywords() for
METH_FASTCALL, avoid the creation of a temporary dictionary for keyword
arguments.

Cleanup also _PyCFunction_FastCallDict():

* Don't dereference func before checking that it's not NULL
* Move code to raise the "no keyword argument" exception into a new
  no_keyword_error label.

Update python-gdb.py for the change.