Refactor code in PyCFunction_Call giving a modest (tiny) speed boost,
a slight improvement in semantics (now detects invalid flag combinations),
and (arguably) improved clarity (making it blindingly clear which flag
combinations are allowed).  All this comes at a cost of a few lines of
code duplication.

* Folded test for METH_KEYWORDS into the switch/case.
* Deferred testing for an empty dictionary until when and where needed.
* Make a similar deferral for filling the "size" variable.
* Inverted the dictionary test so that the common case falls though
  instead of making a jump.