Add parallel compile from pybind11

[wiredfool]

Fixes #8971 .

Changes proposed in this pull request:

• Use the ParallelCompile from pybind11 to compile the c extension in parallel.

With:

$ make clean && time make install
...
real	0m9.226s
user	0m33.522s
sys	0m3.639s

$ make clean && time make debug
...
real	0m4.724s
user	0m15.480s
sys	0m2.941s

On main:

$ make clean && time make install
...
real	0m21.224s
user	0m20.892s
sys	0m2.613s

$ make clean && time make debug
...
real	0m10.243s
user	0m9.450s
sys	0m2.123s

[radarhere]

Suggested change

	ParallelCompile("MAX_CONCURRENCY").install()
	ParallelCompile("MAX_CONCURRENCY", default=4).install()

Actually, from our documentation, this should be 4.

https://pillow.readthedocs.io/en/stable/installation/building-from-source.html#build-options

Config setting: -C parallel=n. Can also be given with environment variable: MAX_CONCURRENCY=n. Pillow can use multiprocessing to build the extension. Setting -C parallel=n sets the number of CPUs to use to n, or can disable parallel building by using a setting of 1. By default, it uses 4 CPUs, or if 4 are not available, as many as are present.

[radarhere]

I've created wiredfool#12 to use -C parallel=n as well.

[hugovk]

From pybind11 docs:

You can also pass default=N to set the default number of threads (0 will take the number of threads available) and max=N, the maximum number of threads; if you have a large extension you may want set this to a memory dependent number.

If not set, pybind11 uses 0.

Pillow's limit of 4 was added in 2014 (#1043) but machines have more CPUs today.

Last I looked, the lowest amount on a macOS on the market was 8, and Linux laptops/desktops usually have at least around this many. Servers and shared systems may be lower.

Shall we leave the default to 0 (and update the docs)?

[radarhere]

Ok, I've pushed a commit.

[hugovk]

Nice speedup.

One concern is pybind11 might not support new Python versions until after we want to start testing.

For example, pybind11 added 3.14 support after beta 1 in May (pybind/pybind11#5646), but we'd already started testing after alpha 3 in January (#8690).

But we can address this when it comes up next time: if pybind11 doesn't support 3.15 when we want to test it, we can make it conditional or temporarily remove it.

[hugovk]

From pybind11 docs:

You can also pass default=N to set the default number of threads (0 will take the number of threads available) and max=N, the maximum number of threads; if you have a large extension you may want set this to a memory dependent number.

If not set, pybind11 uses 0.

Pillow's limit of 4 was added in 2014 (#1043) but machines have more CPUs today.

Last I looked, the lowest amount on a macOS on the market was 8, and Linux laptops/desktops usually have at least around this many. Servers and shared systems may be lower.

Shall we leave the default to 0 (and update the docs)?