gh-137855: Lazy import inspect module in dataclasses

[danielhollas]

inspect module is slow to import (see #117865) and is dragging down dataclasses with it.

There are currently only two uses of inspect in dataclasses, but they are a bit tricky to inline since they are on a direct code path when the @dataclass decorator is executed.

1. inspect.signature is used to autogenerate class docstring (if one is not provided already)
2. inspect.unwrap is used in a rather esoteric code path only for slotted classes, added in gh-90562: Improve zero argument support for super() in dataclasses when slots=True #124692)

For 1. I have used a descriptor protocol to generate the __doc__ attribute on demand (this is my first time messing with descriptors, apologies if I overlooked something).

For 2. can be defferred by calling the unwrap functions only when really necessary (and hopefully this path is not common)

Benchmarks

./python -Ximporttime -c "import dataclasses"

Before

❯ hyperfine -w 10 './python -c "import dataclasses"' 
Benchmark 1: ./python -c "import dataclasses"
  Time (mean ± σ):      19.2 ms ±   2.1 ms    [User: 14.9 ms, System: 4.1 ms]
  Range (min … max):    16.6 ms …  24.1 ms    154 runs

After

❯ hyperfine -w 10 './python -c "import dataclasses"' 
Benchmark 1: ./python -c "import dataclasses"
  Time (mean ± σ):      15.2 ms ±   1.8 ms    [User: 11.2 ms, System: 3.7 ms]
  Range (min … max):    12.9 ms …  19.4 ms    212 runs

Overall seems to be a solid 20-30% improvement.

TODO (if things done here seem acceptable):

• Add blurb
• Add test to guard against import regressions, using ensure_lazy_imports test fixture

• Issue: Improve import time of various stdlib modules #137855

[danielhollas]

This check was copied from the while loop in inspect.unwrap

[danielhollas]

Deferring the call to inspect.signature to create __doc__ might also speed up the dataclasses creation a bit. e.g. here's what I see in a cProfile of the _colorize module which uses dataclasses heavily (on main branch)

$ ./python -m cProfile -m _colorize | head 20
         5604 function calls (5484 primitive calls) in 0.007 seconds

   Ordered by: cumulative time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
      9/1    0.004    0.000    0.007    0.007 {built-in method builtins.exec}
        1    0.000    0.000    0.007    0.007 <string>:1(<module>)
        1    0.000    0.000    0.007    0.007 <frozen runpy>:199(run_module)
        1    0.000    0.000    0.007    0.007 <frozen runpy>:65(_run_code)
        1    0.000    0.000    0.007    0.007 _colorize.py:1(<module>)
        7    0.000    0.000    0.006    0.001 dataclasses.py:1432(wrap)
        7    0.000    0.000    0.006    0.001 dataclasses.py:986(_process_class)
        7    0.000    0.000    0.004    0.001 dataclasses.py:478(add_fns_to_class)
        4    0.000    0.000    0.001    0.000 inspect.py:3343(signature)
        4    0.000    0.000    0.001    0.000 inspect.py:3056(from_callable)
     12/4    0.000    0.000    0.001    0.000 inspect.py:2437(_signature_from_callable)
        4    0.000    0.000    0.001    0.000 inspect.py:2331(_signature_from_function)
    71/11    0.000    0.000    0.000    0.000 annotationlib.py:907(get_annotations)
        8    0.000    0.000    0.000    0.000 dataclasses.py:541(__annotate__)
     17/9    0.000    0.000    0.000    0.000 annotationlib.py:1114(_get_and_call_annotate)

[StanFromIreland]

PEP 810 is landing in 3.15, there is no point in adding an intermediate stage here, we can wait a few weeks.