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>

HISTORY.md

@@ -5,6 +5,8 @@ dev
 ---
 
 -   \[Short description of non-trivial change.\]
+- Requests Brotli compression, if either the `brotli` or `brotlicffi` package
+  is installed.
 
 **Dependencies**