py-urllib3: updated to 1.26.1

1.26.1 (2020-11-11)
-------------------
* Fixed an issue where two ``User-Agent`` headers would be sent if a
  ``User-Agent`` header key is passed as ``bytes``

1.26.0 (2020-11-10)
-------------------
* **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
* 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``
  **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=...)``
* Added default ``User-Agent`` header to every request
* Added ``urllib3.util.SKIP_HEADER`` for skipping ``User-Agent``, ``Accept-Encoding``,
  and ``Host`` headers from being automatically emitted with requests
* Collapse ``transfer-encoding: chunked`` request data and framing into
  the same ``socket.send()`` call
* Send ``http/1.1`` ALPN identifier with every TLS handshake by default
* Properly terminate SecureTransport connections when CA verification fails
* Don't emit an ``SNIMissingWarning`` when passing ``server_hostname=None``
  to SecureTransport
* Disabled requesting TLSv1.2 session tickets as they weren't being used by urllib3
* Suppress ``BrokenPipeError`` when writing request body after the server
  has closed the socket
* Wrap ``ssl.SSLError`` that can be raised from reading a socket (e.g. "bad MAC")
  into an ``urllib3.exceptions.SSLError``

(adam)