build(deps): bump mypy from 1.11.1 to 1.15.0 in /lib/spack/docs

Bumps [mypy](https://github.com/python/mypy) from 1.11.1 to 1.15.0.
- [Changelog](https://github.com/python/mypy/blob/master/CHANGELOG.md)
- [Commits](python/mypy@v1.11.1...v1.15.0)

---
updated-dependencies:
- dependency-name: mypy
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

lib/spack/docs/requirements.txt

@@ -10,4 +10,4 @@ pytest==8.3.4
 isort==5.13.2
 black==24.10.0
 flake8==7.1.1
-mypy==1.11.1
+mypy==1.15.0