ci: Remove brotli from requirements-testing.txt (#6614)

Remove the package from `requirements-testing.txt` and add it only as a dependency of the test suites that rely on it.
This is part of environment cleanup before moving the remaining testing dependencies to a uv dependency group.

Author: alexander-alderman-webb

requirements-testing.txt

@@ -11,5 +11,4 @@ pysocks
 socksio
 httpcore[http2]
 setuptools
-Brotli
 docker

scripts/populate_tox/tox.jinja

@@ -79,6 +79,7 @@ deps =
 
     # === Common ===
     py3.8-common: hypothesis
+    common: brotli
     common: pytest-asyncio
     common: httpcore[asyncio]
     # See https://github.com/pytest-dev/pytest/issues/9621
@@ -97,6 +98,7 @@ deps =
     # for justification of the upper bound on pytest
     {py3.6,py3.7}-gevent: pytest<7.0.0
     {py3.8,py3.9,py3.10,py3.11,py3.12}-gevent: pytest
+    gevent: brotli
     gevent: pytest-asyncio
     gevent: setuptools<82
     {py3.10,py3.11}-gevent: zope.event<5.0.0

tests/conftest.py

@@ -13,7 +13,11 @@
 from unittest import mock
 from urllib.parse import parse_qs, urlparse
 
-import brotli
+try:
+    import brotli
+except ImportError:
+    brotli = None
+
 import jsonschema
 import pytest
 from pytest_localserver.http import WSGIServer
@@ -1641,6 +1645,7 @@ def __call__(self, environ, start_response):
             rdr = gzip.GzipFile(fileobj=io.BytesIO(request.data))
             compressed = True
         elif content_encoding == "br":
+            assert brotli is not None
             rdr = io.BytesIO(brotli.decompress(request.data))
             compressed = True
         else: