chore: use if TYPE_CHECKING: blocks

[mayeut]

This will become the default in Python 3.14 in any case given the current lint rules.
It ensures typing only imports do not slow down execution (admittedly only useful in edge use cases, such as querying help).

If #2797 makes it, it will further speed-up this one:

hyperfine --warmup 10 \
     -n "main"                --prepare "git checkout main"               ".nox/tests-3-15/bin/python -m cibuildwheel --help" \
     -n "Henry's PR"          --prepare "git checkout pr/2797"            ".nox/tests-3-15/bin/python -m cibuildwheel --help" \
     -n "type-checking"       --prepare "git checkout type-checking"      ".nox/tests-3-15/bin/python -m cibuildwheel --help" \
     -n "type-checking-lazy"  --prepare "git checkout type-checking-lazy" ".nox/tests-3-15/bin/python -m cibuildwheel --help"
Benchmark 1: main
  Time (mean ± σ):     308.7 ms ±  16.7 ms    [User: 194.7 ms, System: 44.1 ms]
  Range (min … max):   294.3 ms … 349.3 ms    10 runs
 
Benchmark 2: Henry's PR
  Time (mean ± σ):     106.2 ms ±   9.4 ms    [User: 62.7 ms, System: 17.6 ms]
  Range (min … max):    88.7 ms … 118.2 ms    19 runs
 
Benchmark 3: type-checking (this PR)
  Time (mean ± σ):     306.9 ms ±  15.5 ms    [User: 194.2 ms, System: 43.2 ms]
  Range (min … max):   290.2 ms … 342.8 ms    10 runs
 
Benchmark 4: type-checking-lazy
  Time (mean ± σ):      77.5 ms ±   6.1 ms    [User: 45.6 ms, System: 13.7 ms]
  Range (min … max):    68.9 ms …  86.3 ms    22 runs
 
Summary
  type-checking-lazy ran
    1.37 ± 0.16 times faster than Henry's PR
    3.96 ± 0.37 times faster than type-checking (this PR)
    3.98 ± 0.38 times faster than main

[henryiii]

Sounds good to me.

[freakboy3742]

All makes sense to me

[mayeut]

After further thoughts, we might want to:

• re-allow typing.TYPE_CHECKING (use forbidden in chore: avoid importing TYPE_CHECKING #2864) as typing is always imported by cibuildwheel. If PEP 781 makes it, we can forbid typing.TYPE_CHECKING once dropping the Python version before its introduction. Only the fact that there are TYPE_CHECKING blocks is important for fix: faster CLI on Python 3.15 #2797. Whether we use TYPE_CHECKING = False or typing.TYPE_CHECKING doesn't change the fact that typing is always imported.
• disable TC checks for tests where there's no added value in timings at all I think

Any thoughts ?

[mayeut]

I've implemented the proposal from the previous comment in #2869
Changed the status to draft until there's a consensus on which PR to keep if any.

[henryiii]

I like this version fine - TYPE_CHECKING = False is accepted by all the type checkers (including the new Rust ones), and is the only proper workaround that works with lazy imports too until PEP 781 makes it. I'd be fine to see it everywhere, might help get PEP 781 in. :)

In timings, typing forces an re import, which is quite expensive.

I'm fine to keep tests consistent, though I don't really care, we don't have to.

[joerick]

I don't have a strong opinion, but the TYPE_CHECKING = False thing is fine by me!

[mayeut]

In timings, typing forces an re import, which is quite expensive.

Just as a note, typing is always imported by cibuildwheel - at least for now.

I closed the other PR and kept this one as I don't have a strong opinion either.