COMP: Bootstrap pip via ensurepip on Windows wheel build to fix Python 3.10+

[hjmjohnson]

Bootstrap pip via python -m ensurepip --upgrade --default-pip before invoking the system pip in windows_build_module_wheels.py. Sidesteps a Python 3.10+ chicken-and-egg failure where the system pip's vendored html5lib does from collections import Mapping (removed in 3.10) and dies before any pip install can run.

Surfaced by KitwareMedical/ITKUltrasound#246.

Failure trace

File ".../Temp/tmpXXXX/pip.zip/pip/_vendor/html5lib/_trie/_base.py", line 3
ImportError: cannot import name 'Mapping' from 'collections' (C:\Python310-x64\lib\collections\__init__.py)

Consistent on Python 3.10 and 3.11 across all build-windows-python-packages matrix jobs.

Root cause

scripts/windows-download-cache-and-build-module-wheels.ps1 IEXes the upstream install-python.ps1 from scikit-build/scikit-ci-addons. That script's line 160 downloads get-pip.py from a pinned GitHub gist (jcfr/db7347e8708b9f32d45ab36125fad6d3@8478d43e) rather than https://bootstrap.pypa.io/get-pip.py. The pinned gist installs a pip old enough to vendor an html5lib that still uses from collections import Mapping.

When windows_build_module_wheels.py:55 runs python -m pip install pip --upgrade, the system pip loads its _vendor/html5lib._trie._base and dies before any package-resolution logic runs. The bug cannot be worked around using the broken pip itself.

Why ensurepip works

ensurepip installs the pip wheel that CPython itself bundled at release time (Lib/ensurepip/_bundled/), which is ≥ 21.x for Python 3.10 and ≥ 23.x for 3.11 — both ship a working html5lib ≥ 1.1 with the collections.abc.Mapping migration applied. Once ensurepip writes a fresh pip into the system Python, the subsequent pip install pip --upgrade finds a working pip and proceeds normally.

What is NOT changed

• windows-download-cache-and-build-module-wheels.ps1: still IEXes the upstream install-python.ps1 unchanged. The fix is purely Python-side; no scikit-ci-addons interaction needed.
• scripts/windows_build_wheels.py: unchanged. Its pip operations go through a virtualenv created by virtualenv.exe, and modern virtualenv ≥ 20.x ships its own bundled pip seed wheels instead of copying the (broken) system pip into the new venv.
• The upstream scikit-ci-addons pinned gist URL: ideally fixed at the source eventually (replace with bootstrap.pypa.io/get-pip.py), but that's a separate, broader-scope change in a different repo.

Test plan

• Windows wheel build (Python 3.10) on this repo's CI passes the bootstrap step
• Windows wheel build (Python 3.11) on this repo's CI passes the bootstrap step
• After merge, COMP: Update ITKRemoteModuleBuildTestPackageAction to v5.4.6 KitwareMedical/ITKUltrasound#246's build-windows-python-packages (10) and (11) jobs progress past pip install into actual C++ compilation