Speed up import time by deferring inspect

[danielhollas]

Deferring import of inspect cuts the import time by ~10% (4ms on my machine).

CPython issue: https://github.com/python/cpython/issues/118761

I haven't been able to run the tests locally, seeing these errors:

RROR tests/test_api.py - AttributeError: module 'tests.fixtures' has no attribute 'EggInfoPkg'
ERROR tests/test_integration.py - AttributeError: module 'tests.fixtures' has no attribute 'DistInfoPkg'
ERROR tests/test_main.py
ERROR tests/test_zip.py - AttributeError: module 'tests.fixtures' has no attribute 'ZipFixtures'

Benchmarks

These have been run with latest CPython main branch (as of Aug 6th 2024), these gains are likely representative for Python 3.13, but not 3.12.

this PR

❯ hyperfine -w 5 'python -c "import importlib_metadata"'
Benchmark 1: python -c "import importlib_metadata"
  Time (mean ± σ):      39.6 ms ±   4.6 ms    [User: 29.8 ms, System: 9.4 ms]
  Range (min … max):    29.7 ms …  49.8 ms    68 runs

main

❯ hyperfine -w 5 'python -c "import importlib_metadata"'
Benchmark 1: python -c "import importlib_metadata"
  Time (mean ± σ):      43.4 ms ±   3.6 ms    [User: 33.1 ms, System: 9.8 ms]
  Range (min … max):    34.9 ms …  50.1 ms    68 runs

[jaraco]

I've minimized the change to address the specific issue. I'll deal with the typeshed ignore workaround separately.

[jaraco]

I should have added a news fragment before merging. I added it later in 71b4678432.

[jaraco]

This change is released in v8.4.0.