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 deferred 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.

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