build(deps): bump urllib3 from 1.26.15 to 1.26.18 in /docs/sphinx (#1508)

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.15 to
1.26.18.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.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>
<h2>1.26.17</h2>
<ul>
<li>Added the <code>Cookie</code> header to the list of headers to strip
from requests when redirecting to a different host. As before, different
headers can be set via <code>Retry.remove_headers_on_redirect</code>.
(GHSA-v845-jxx5-vc9f)</li>
</ul>
<h2>1.26.16</h2>
<ul>
<li>Fixed thread-safety issue where accessing a <code>PoolManager</code>
with many distinct origins would cause connection pools to be closed
while requests are in progress (<a
href="https://redirect.github.com/urllib3/urllib3/issues/2954">#2954</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a
href="https://github.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>
<h1>1.26.17 (2023-10-02)</h1>
<ul>
<li>Added the <code>Cookie</code> header to the list of headers to strip
from requests when redirecting to a different host. As before, different
headers can be set via <code>Retry.remove_headers_on_redirect</code>.
(<code>[#3139](https://github.com/urllib3/urllib3/issues/3139)
&lt;https://github.com/urllib3/urllib3/pull/3139&gt;</code>_)</li>
</ul>
<h1>1.26.16 (2023-05-23)</h1>
<ul>
<li>Fixed thread-safety issue where accessing a <code>PoolManager</code>
with many distinct origins
would cause connection pools to be closed while requests are in progress
(<code>[#2954](https://github.com/urllib3/urllib3/issues/2954)
&lt;https://github.com/urllib3/urllib3/pull/2954&gt;</code>_)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.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://github.com/urllib3/urllib3/commit/b594c5ceaca38e1ac215f916538fb128e3526a36"><code>b594c5c</code></a>
Merge pull request from GHSA-g4mx-q9vg-27p4</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/944f0eb134485f41bc531be52de12ba5a37bca73"><code>944f0eb</code></a>
[1.26] Use vendored six in urllib3.contrib.securetransport</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/c9016bf464751a02b7e46f8b86504f47d4238784"><code>c9016bf</code></a>
Release 1.26.17</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/01220354d389cd05474713f8c982d05c9b17aafb"><code>0122035</code></a>
Backport GHSA-v845-jxx5-vc9f (<a
href="https://redirect.github.com/urllib3/urllib3/issues/3139">#3139</a>)</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/e63989f97d206e839ab9170c8a76e3e097cc60e8"><code>e63989f</code></a>
Fix installing <code>brotli</code> extra on Python 2.7</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/2e7a24d08713a0131f0b3c7197889466d645cc49"><code>2e7a24d</code></a>
[1.26] Configure OS for RTD to fix building docs</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/57181d6ea910ac7cb2ff83345d9e5e0eb816a0d0"><code>57181d6</code></a>
[1.26] Improve error message when calling urllib3.request() (<a
href="https://redirect.github.com/urllib3/urllib3/issues/3058">#3058</a>)</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/3c0148048a523325819377b23fc67f8d46afc3aa"><code>3c01480</code></a>
[1.26] Run coverage even with failed jobs</li>
<li><a
href="https://github.com/urllib3/urllib3/commit/d94029b7e2193ff47b627906a70e06377a09aae8"><code>d94029b</code></a>
Release 1.26.16</li>
<li>Additional commits viewable in <a
href="https://github.com/urllib3/urllib3/compare/1.26.15...1.26.18">compare
view</a></li>
</ul>
</details>
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