Add frozendict support to pickle

[rhettinger]

>>> from pickle import dumps
>>> dumps(frozendict(a=1, b=2))
Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    dumps(frozendict(a=1, b=2))
TypeError: cannot pickle 'frozendict' object

[brianschubert]

Added in #144967, this works on main:

>>> from pickle import dumps
>>> dumps(frozendict(a=1, b=2))
b'\x80\x05\x95/\x00\x00\x00\x00\x00\x00\x00\x8c\x08builtins\x94\x8c\nfrozendict\x94\x93\x94}\x94(\x8c\x01a\x94K\x01\x8c\x01b\x94K\x02u\x85\x94\x81\x94.'