gh-154675: Avoid importing inspect when creating a slotted dataclass

[aisk]

Using @dataclass(slots=True) always imports inspect, just for the single inspect.unwrap() call in _add_slots().

typing and annotationlib already inline the same __wrapped__ walk to avoid the import, so dataclasses can too. Perhaps in the future unwrap() could move into a small private module that inspect and all these copies import, so the implementations do not drift apart.

Importing a module that defines one slotted dataclass, best of 30 runs.

$ cat app.py
from dataclasses import dataclass

@dataclass(slots=True)
class Point:
    x: int = 0
    y: int = 0

$ ./python -c 'import time; t = time.perf_counter(); import app; print(time.perf_counter() - t)'
0.022856871997646522   # before
0.010011243997723795   # after

About 2.3x faster.

• Issue: Improve import time of various stdlib modules #154675