Bump pip from 21.0 to 25.2 in /.github/workflows

Bumps [pip](https://github.com/pypa/pip) from 21.0 to 25.2.
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@21.0...25.2)

---
updated-dependencies:
- dependency-name: pip
  dependency-version: '25.2'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
-pip==21.0
+pip==25.2
 pre-commit==2.9.3
 black==20.8b1
 flake8==3.8.4