Bump actions/cache from 2.1.5 to 2.1.6 (#599)

Bumps [actions/cache](https://github.com/actions/cache) from 2.1.5 to 2.1.6.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/actions/cache/releases">actions/cache's releases</a>.</em></p>
<blockquote>
<h2>v2.1.6</h2>
<ul>
<li>Catch unhandled &quot;bad file descriptor&quot; errors that sometimes occurs when the cache server returns non-successful response (<a href="https://github-redirect.dependabot.com/actions/cache/pull/596">actions/cache#596</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/actions/cache/commit/c64c572235d810460d0d6876e9c705ad5002b353"><code>c64c572</code></a> Catch and log unhandled exceptions stemming from closed file descriptor (<a href="https://github-redirect.dependabot.com/actions/cache/issues/596">#596</a>)</li>
<li><a href="https://github.com/actions/cache/commit/cc2d767a726ddd654506e76169ec14b20dc75915"><code>cc2d767</code></a> Update Rust directories recommended for caching (<a href="https://github-redirect.dependabot.com/actions/cache/issues/433">#433</a>)</li>
<li><a href="https://github.com/actions/cache/commit/2fa955d825a29821e73ec064792122ea9f70e573"><code>2fa955d</code></a> Update examples.md (<a href="https://github-redirect.dependabot.com/actions/cache/issues/588">#588</a>)</li>
<li><a href="https://github.com/actions/cache/commit/3a696372f2ec10535e4c026a4fdae5fcca629e4a"><code>3a69637</code></a> elixir typo - stray parenthesis (<a href="https://github-redirect.dependabot.com/actions/cache/issues/569">#569</a>)</li>
<li><a href="https://github.com/actions/cache/commit/366e5ba0224e0ebd6fba2a453d108cb20e734946"><code>366e5ba</code></a> Update cache key for Elixir (<a href="https://github-redirect.dependabot.com/actions/cache/issues/568">#568</a>)</li>
<li><a href="https://github.com/actions/cache/commit/8d3f2fc3ce25d6c95a1297ce8d93f9c3c3ac8f00"><code>8d3f2fc</code></a> Update dependencies (<a href="https://github-redirect.dependabot.com/actions/cache/issues/565">#565</a>)</li>
<li>See full diff in <a href="https://github.com/actions/cache/compare/v2.1.5...v2.1.6">compare view</a></li>
</ul>
</details>
<br />


[![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=actions/cache&package-manager=github_actions&previous-version=2.1.5&new-version=2.1.6)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores)

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`.

[//]: # (dependabot-automerge-start)
[//]: # (dependabot-automerge-end)

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
       with:
         python-version: 3.9
     - name: Cache PyPI
-      uses: actions/[email protected].5
+      uses: actions/[email protected].6
       with:
         key: pip-lint-${{ hashFiles('requirements/*.txt') }}
         path: ~/.cache/pip
@@ -90,7 +90,7 @@ jobs:
       run: |
         echo "::set-output name=dir::$(pip cache dir)"    # - name: Cache
     - name: Cache PyPI
-      uses: actions/[email protected].5
+      uses: actions/[email protected].6
       with:
         key: pip-ci-${{ runner.os }}-${{ matrix.pyver }}-{{ matrix.no-extensions }}-${{ hashFiles('requirements/*.txt') }}
         path: ${{ steps.pip-cache.outputs.dir }}