Update urllib3 to 1.26.3

[pyup-bot]

This PR updates urllib3 from 1.25.11 to 1.26.3.

Changelog

1.26.2

-------------------

* Fixed an issue where ``wrap_socket`` and ``CERT_REQUIRED`` wouldn't
be imported properly on Python 2.7.8 and earlier (Pull 2052)

1.26.1

-------------------

* Fixed an issue where two ``User-Agent`` headers would be sent if a
``User-Agent`` header key is passed as ``bytes`` (Pull 2047)

1.26.0

-------------------

* **NOTE: urllib3 v2.0 will drop support for Python 2**.
`Read more in the v2.0 Roadmap <https://urllib3.readthedocs.io/en/latest/v2-roadmap.html>`_.

* Added support for HTTPS proxies contacting HTTPS servers (Pull 1923, Pull 1806)

* Deprecated negotiating TLSv1 and TLSv1.1 by default. Users that
still wish to use TLS earlier than 1.2 without a deprecation warning
should opt-in explicitly by setting ``ssl_version=ssl.PROTOCOL_TLSv1_1`` (Pull 2002)
**Starting in urllib3 v2.0: Connections that receive a ``DeprecationWarning`` will fail**

* Deprecated ``Retry`` options ``Retry.DEFAULT_METHOD_WHITELIST``, ``Retry.DEFAULT_REDIRECT_HEADERS_BLACKLIST``
and ``Retry(method_whitelist=...)`` in favor of ``Retry.DEFAULT_ALLOWED_METHODS``,
``Retry.DEFAULT_REMOVE_HEADERS_ON_REDIRECT``, and ``Retry(allowed_methods=...)``
(Pull 2000) **Starting in urllib3 v2.0: Deprecated options will be removed**

* Added default ``User-Agent`` header to every request (Pull 1750)

* Added ``urllib3.util.SKIP_HEADER`` for skipping ``User-Agent``, ``Accept-Encoding``, 
and ``Host`` headers from being automatically emitted with requests (Pull 2018)

* Collapse ``transfer-encoding: chunked`` request data and framing into
the same ``socket.send()`` call (Pull 1906)

* Send ``http/1.1`` ALPN identifier with every TLS handshake by default (Pull 1894)

* Properly terminate SecureTransport connections when CA verification fails (Pull 1977)

* Don't emit an ``SNIMissingWarning`` when passing ``server_hostname=None``
to SecureTransport (Pull 1903)

* Disabled requesting TLSv1.2 session tickets as they weren't being used by urllib3 (Pull 1970)

* Suppress ``BrokenPipeError`` when writing request body after the server
has closed the socket (Pull 1524)

* Wrap ``ssl.SSLError`` that can be raised from reading a socket (e.g. "bad MAC")
into an ``urllib3.exceptions.SSLError`` (Pull 1939)

Links

• PyPI: https://pypi.org/project/urllib3
• Changelog: https://pyup.io/changelogs/urllib3/
• Docs: https://urllib3.readthedocs.io/