feat: add CPython 3.15 support for iOS (#2857)

* feat: add CPython 3.15 support for iOS

Assisted-by: OpenCode:Kimi-K2.6
Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

* fix: copy in iOS support files

Assisted-by: Copilot:claude-sonnet-4.6
Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

* fix: skip directories when copying iOS support files

When copying multiarch-specific support files (e.g. _cross_arm64_iphoneos.py),
the code was trying to copy all items in the directory including __pycache__
directories. The shutil.copy() function only works with files, not directories,
which caused an IsADirectoryError.

This fix adds a check to only copy files, skipping any directories like
__pycache__ that may have been created by Python imports.

Assisted-by: Copilot:claude-haiku-4.5

* fix: - in dir fine for now

Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

* fix: apply review suggestions

Assisted-by: Copilot:claude-sonnet-4.6
Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

* fix: only do this on 3.15+

Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

* Ensure target Python directory exists before build

Add assertion to check if target Python directory exists.

* fix(ios): use stdlib dir directly instead of copying to platform-config

Remove the _inject_support_files workaround for python.org 3.15+
distributions. Instead of copying sysconfig files from the stdlib into
a synthetic platform-config/ directory, pass the stdlib directory
directly to make_cross_venv.py.

make_cross_venv.py is updated to derive the multiarch tag from the
_sysconfigdata_ filename rather than assuming it comes from the
directory name.

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>
Assisted-by: OpenCode:Kimi-K2.6
Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

* Minor format cleanups on pre-commit skipped files.

* Clarify path names in make_cross_venv script.

---------

Signed-off-by: Henry Schreiner <henryfs@princeton.edu>
Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>

.pre-commit-config.yaml

@@ -1,5 +1,6 @@
 ci:
   autoupdate_schedule: monthly
+exclude: "^cibuildwheel/resources/ios-support/(?!make_cross_venv\\.py)"
 
 repos:
 - repo: https://github.com/pre-commit/pre-commit-hooks