gh-155258: Make the curses.complexstr signature introspectable

[fedonman]

complexstr's two optional parameters were declared object = NULL, and NULL has no
Python repr, so Argument Clinic wrote <unrepresentable> into the text signature and
inspect.signature() raised.

Declaring them object(c_default="NULL") = None makes the default representable and
leaves the C default as NULL, so "omitted" and "given" stay distinguishable:

before:
  complexstr:  ValueError: <class 'curses.complexstr'> builtin has invalid signature
  __text_signature__: (cells, /, attr=<unrepresentable>, pair=<unrepresentable>)
after:
  signature: (cells, /, attr=None, pair=None)

Behaviour does not change. The six constructor forms that pass or omit attr and pair,
with a string and with a sequence, give identical results before and after, including the
three that raise. 37 other stdlib parameters already use this declaration.

test_curses passes under TERM=xterm-256color and vt100. The new assertion fails
without the change, with the ValueError above.

complexstr is new in 3.16, so there is no NEWS entry.

• Issue: inspect.signature() fails on the new curses.complexstr type #155258