Bump tox from 4.20.0 to 4.21.0 (#420)

Bumps [tox](https://github.com/tox-dev/tox) from 4.20.0 to 4.21.0.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a
href="https://github.com/tox-dev/tox/blob/main/docs/changelog.rst">tox's
changelog</a>.</em></p>
<blockquote>
<h2>v4.21.0 (2024-09-30)</h2>
<p>Features - 4.21.0</p>
<pre><code>- Native TOML configuration support - by :user:`gaborbernat`.
(:issue:`999`)
<p>Improved Documentation - 4.21.0
</code></pre></p>
<ul>
<li>Update Loader docs - by :user:ziima (:issue:<code>3352</code>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/tox-dev/tox/commit/46bef9eff54c53eeced031436d2d1a068b4bfb16"><code>46bef9e</code></a>
release 4.21.0</li>
<li><a
href="https://github.com/tox-dev/tox/commit/cea105b7007039c830c92ae92aa5792e819ee487"><code>cea105b</code></a>
True TOML config support (<a
href="https://redirect.github.com/tox-dev/tox/issues/3353">#3353</a>)</li>
<li><a
href="https://github.com/tox-dev/tox/commit/a04b039fe0486752ec79733b993d73ecdf282795"><code>a04b039</code></a>
[pre-commit.ci] pre-commit autoupdate (<a
href="https://redirect.github.com/tox-dev/tox/issues/3385">#3385</a>)</li>
<li><a
href="https://github.com/tox-dev/tox/commit/f5eba31ed3aeedcb41db64fca5b624a75abff32d"><code>f5eba31</code></a>
Update Loader docs (<a
href="https://redirect.github.com/tox-dev/tox/issues/3352">#3352</a>)</li>
<li><a
href="https://github.com/tox-dev/tox/commit/329db639b5573cf9b5990fe990a3e48789d1fc9a"><code>329db63</code></a>
[pre-commit.ci] pre-commit autoupdate (<a
href="https://redirect.github.com/tox-dev/tox/issues/3350">#3350</a>)</li>
<li><a
href="https://github.com/tox-dev/tox/commit/f4b3bd20f268e6da78c2a89b531f364cec35b90b"><code>f4b3bd2</code></a>
Bump pypa/gh-action-pypi-publish from 1.10.1 to 1.10.2 (<a
href="https://redirect.github.com/tox-dev/tox/issues/3349">#3349</a>)</li>
<li>See full diff in <a
href="https://github.com/tox-dev/tox/compare/4.20.0...4.21.0">compare
view</a></li>
</ul>
</details>
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