Add support for brotli decoding (#5783)

* Add support for Brotli decoding

When the brotli or brotlicffi packages are installed,
urllib3.util.make_headers() inserts ',br' in the Accept-Encoding header
and decodes br from the answers.

* Create the default Accept-Encoding header once

* Preserve the previous delimiter behavior

* Update prose in quickstart.rst

Co-authored-by: Seth Michael Larson <sethmichaellarson@gmail.com>

docs/community/faq.rst

@@ -11,6 +11,9 @@ Encoded Data?
 Requests automatically decompresses gzip-encoded responses, and does
 its best to decode response content to unicode when possible.
 
+When either the `brotli <https://pypi.org/project/Brotli/>`_ or `brotlicffi <https://pypi.org/project/brotlicffi/>`_
+package is installed, requests also decodes Brotli-encoded responses.
+
 You can get direct access to the raw response (and even the socket),
 if needed as well.
 

docs/user/quickstart.rst

@@ -128,6 +128,9 @@ You can also access the response body as bytes, for non-text requests::
 
 The ``gzip`` and ``deflate`` transfer-encodings are automatically decoded for you.
 
+The ``br``  transfer-encoding is automatically decoded for you if a Brotli library
+like `brotli <https://pypi.org/project/brotli>`_ or `brotlicffi <https://pypi.org/project/brotli>`_ is installed.
+
 For example, to create an image from binary data returned by a request, you can
 use the following code::