Update urllib3 to 1.26.0

[pyup-bot]

This PR updates urllib3 from 1.25.10 to 1.26.0.

Changelog

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)

1.25.11

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

* Fix retry backoff time parsed from ``Retry-After`` header when given
in the HTTP date format. The HTTP date was parsed as the local timezone
rather than accounting for the timezone in the HTTP date (typically
UTC) (Pull 1932, Pull 1935, Pull 1938, Pull 1949)

* Fix issue where an error would be raised when the ``SSLKEYLOGFILE``
environment variable was set to the empty string. Now ``SSLContext.keylog_file``
is not set in this situation (Pull 2016)

Links

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

[pyup-bot]

Closing this in favor of #2428