Bump actions/cache from 2.1.5 to 2.1.6 (#96)

Bumps [actions/cache](https://github.com/actions/cache) from 2.1.5 to 2.1.6.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](actions/cache@v2.1.5...v2.1.6)

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

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/tests.yaml

@@ -36,7 +36,7 @@ jobs:
           pip install --constraint=.github/workflows/constraints.txt poetry
 
       - name: Enable caching
-        uses: actions/[email protected].5
+        uses: actions/[email protected].6
         with:
           path: ~/.cache/pypoetry/virtualenvs
           key: ${{ runner.os }}-poetry-${{ hashFiles('poetry.lock') }}