<h3>Contribution description</h3>
<p>Since I'm using mypy to lint python files, it's cache directory shows up in "git status".<br>
This PR adds that folder to .gitignore.</p>

<h3>Testing procedure</h3>

<h3>Issues/PRs references</h3>


<hr>

<h4>You can view, comment on, or merge this pull request online at:</h4>
<p>  <a href='https://github.com/RIOT-OS/RIOT/pull/11764'>https://github.com/RIOT-OS/RIOT/pull/11764</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>.gitignore: add .mypy_cache/</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/11764/files#diff-0">.gitignore</a>
    (3)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/11764.patch'>https://github.com/RIOT-OS/RIOT/pull/11764.patch</a></li>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/11764.diff'>https://github.com/RIOT-OS/RIOT/pull/11764.diff</a></li>
</ul>

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