gh-125028: Prohibit placeholders in partial keywords

[dg-pb]

Slight initialization penalty for vanilla case, but slightly faster for subclass optimization.

Bench

S="from functools import partial, Placeholderf = lambda a, b: Nonep = partial(f, a=1)"
C0='partial(f, a=1)'
C1='partial(p, b=2)'# BEFORE | AFTER$PYEXE -m timeit -s $S$C0# 180 ns | 210 ns$PYEXE -m timeit -s $S$C1# 280 ns | 250 ns

• Issue: Prohibit placeholders in partial keywords #125028

[dg-pb]

Not planned.