Bump actions/cache from 3 to 4

Bumps [actions/cache](https://github.com/actions/cache) from 3 to 4.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v3...v4)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/pythontests.yml

@@ -25,7 +25,7 @@ jobs:
       run: |
         echo "::set-output name=dir::$(pip cache dir)"
     - name: pip cache
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: ${{ steps.pip-cache.outputs.dir }}
         key: py${{ matrix.python-version }}-${{ matrix.os }}-pip
@@ -58,7 +58,7 @@ jobs:
         python -m pip install -U pip # to ensure version > 20 to have cache dir
         echo "::set-output name=dir::$(pip cache dir)"
     - name: pip cache
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: ${{ steps.pip-cache.outputs.dir }}
         key: py${{ matrix.python-version }}-${{ matrix.os }}-pip