chore(deps): bump urllib3 from 1.26.17 to 1.26.18 in /.kokoro (#150)

Bumps [urllib3](https://togithub.com/urllib3/urllib3) from 1.26.17 to 1.26.18.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://togithub.com/urllib3/urllib3/releases">urllib3's releases</a>.</em></p>
<blockquote>
<h2>1.26.18</h2>
<ul>
<li>Made body stripped from HTTP requests changing the request method to GET after HTTP 303 &quot;See Other&quot; redirect responses. (GHSA-g4mx-q9vg-27p4)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://togithub.com/urllib3/urllib3/blob/main/CHANGES.rst">urllib3's changelog</a>.</em></p>
<blockquote>
<h1>1.26.18 (2023-10-17)</h1>
<ul>
<li>Made body stripped from HTTP requests changing the request method to GET after HTTP 303 &quot;See Other&quot; redirect responses.</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://togithub.com/urllib3/urllib3/commit/9c2c2307dd1d6af504e09aac0326d86ee3597a0b"><code>9c2c230</code></a> Release 1.26.18 (<a href="https://redirect.github.com/urllib3/urllib3/issues/3159">#3159</a>)</li>
<li><a href="https://togithub.com/urllib3/urllib3/commit/b594c5ceaca38e1ac215f916538fb128e3526a36"><code>b594c5c</code></a> Merge pull request from GHSA-g4mx-q9vg-27p4</li>
<li><a href="https://togithub.com/urllib3/urllib3/commit/944f0eb134485f41bc531be52de12ba5a37bca73"><code>944f0eb</code></a> [1.26] Use vendored six in urllib3.contrib.securetransport</li>
<li>See full diff in <a href="https://togithub.com/urllib3/urllib3/compare/1.26.17...1.26.18">compare view</a></li>
</ul>
</details>
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