Bump urllib3 from 1.26.2 to 1.26.3 in /2020#4

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.2 to 1.26.3. <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>1.26.3</h2> <p>:warning: <strong>IMPORTANT: urllib3 v2.0 will drop support for Python 2</strong>: <a href="https://urllib3.readthedocs.io/en/latest/v2-roadmap.html">Read more in the v2.0 Roadmap</a></p> <ul> <li> <p>Fixed bytes and string comparison issue with headers (Pull <a href="https://github-redirect.dependabot.com/urllib3/urllib3/issues/2141">#2141</a>)</p> </li> <li> <p>Changed <code>ProxySchemeUnknown</code> error message to be more actionable if the user supplies a proxy URL without a scheme (Pull <a href="https://github-redirect.dependabot.com/urllib3/urllib3/issues/2107">#2107</a>)</p> </li> </ul> <p><strong>If you or your organization rely on urllib3 consider supporting us via <a href="https://github.com/sponsors/urllib3">GitHub Sponsors</a></strong></p> </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> <h2>1.26.3 (2021-01-26)</h2> <ul> <li> <p>Fixed bytes and string comparison issue with headers (Pull <a href="https://github-redirect.dependabot.com/urllib3/urllib3/issues/2141">#2141</a>)</p> </li> <li> <p>Changed <code>ProxySchemeUnknown</code> error message to be more actionable if the user supplies a proxy URL without a scheme. (Pull <a href="https://github-redirect.dependabot.com/urllib3/urllib3/issues/2107">#2107</a>)</p> </li> </ul> </blockquote> </details> <details> <summary>Commits</summary> <ul> <li><a href="https://github.com/urllib3/urllib3/commit/361f1e2a61afdef86cb2feb0fa3f302e06c5fe2c"><code>361f1e2</code></a> Release 1.26.3</li> <li><a href="https://github.com/urllib3/urllib3/commit/3179dfdac1ca8137fe2d5d480a18642311adf0fb"><code>3179dfd</code></a> Allow using deprecated OpenSSL with CRYPTOGRAPHY_ALLOW_OPENSSL_102</li> <li><a href="https://github.com/urllib3/urllib3/commit/d97e5d4cbd96f21b4d16ae914d5797ac44a6dbdd"><code>d97e5d4</code></a> Use Python 3.5 compatible get-pip</li> <li><a href="https://github.com/urllib3/urllib3/commit/cb5e2fc6af52b226e3ce68414b1afa23416a036d"><code>cb5e2fc</code></a> [1.26] Don't compare bytes and str in putheader()</li> <li><a href="https://github.com/urllib3/urllib3/commit/b89158f30f9fa427ec3cbad29368d917f4fdd367"><code>b89158f</code></a> [1.26] Update RECENT_DATE to 2020-07-01</li> <li><a href="https://github.com/urllib3/urllib3/commit/a800c745969ef7d06ad3ea0a7722da521f2934cd"><code>a800c74</code></a> [1.26] Recommend GitHub Sponsors instead of Open Collective</li> <li><a href="https://github.com/urllib3/urllib3/commit/947284e6bede3fe4e1098844680a36c0389c2c85"><code>947284e</code></a> [1.26] Improve message for ProxySchemeUnknown exception</li> <li>See full diff in <a href="https://github.com/urllib3/urllib3/compare/1.26.2...1.26.3">compare view</a></li> </ul> </details> <br /> [](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. 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