Update bundled pip to 26.3 #161212

Feature or enhancement

Update ensurepip's bundled pip from 26.2.2 to 26.3.

• Release: https://pypi.org/project/pip/26.3/
• Changelog: https://pip.pypa.io/en/stable/news/

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere.

Links to previous discussion of this feature:

Previous update: #155063

Linked PRs

• gh-161212: Upgrade bundled pip to 26.3 #161214
• [3.15] gh-161212: Upgrade bundled pip to 26.3 (GH-161214) #161219
• [3.14] gh-161212: Upgrade bundled pip to 26.3 (GH-161214) #161220