gh-148871: Add CONSTANT_EMPTY_TUPLE to LOAD_COMMON_CONSTANT

[eendebakpt]

Add () as a new value for LOAD_COMMON_CONSTANT, following the existing pattern used for "" (CONSTANT_EMPTY_STR). The compiler now folds LOAD_CONST () into LOAD_COMMON_CONSTANT 12, dropping the entry from co_consts.

() is the most common constant left in co_consts after the existing LOAD_COMMON_CONSTANT rewrites. Frequencies in co_consts (top-level entries, recursing into nested code objects):

Constant	stdlib (613,524 entries)	pyperformance (9,550 entries)	Immortal singleton?
()	14,270 (2.33%)	112 (1.17%)	yes (Py_CONSTANT_EMPTY_TUPLE)
b""	1,012 (0.16%)	4 (0.04%)	yes (Py_CONSTANT_EMPTY_BYTES)
1.0	516 (0.08%)	14 (0.15%)	no (would need new immortal float)
0.0	457 (0.07%)	13 (0.14%)	no (would need new immortal float; care re: -0.0)
...	139 (0.02%)	1 (0.01%)	yes (_Py_EllipsisObject)

Common sources of (): __slots__ = (), empty default-args tuples passed to MAKE_FUNCTION, __reduce__ returning (), etc.

b"" and ... are already immortal singletons, but less frequent. For reviewers: should we add these as well?

• Issue: Extend and improve LOAD_COMMON_CONSTANT #148871

[python-cla-bot]

All commit authors signed the Contributor License Agreement.