diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 10e24684..4fbc305f 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -27,7 +27,7 @@ jobs:
           ${{ runner.os }}-pip-lint-
           ${{ runner.os }}-pip-
 
-    - run: python -m pip install tox
+    - run: python -m pip install 'tox<4'
 
     - run: tox -q -p all -e flake8,towncrier,twine,check-manifest
 
@@ -63,7 +63,7 @@ jobs:
     continue-on-error: ${{ matrix.experimental }}
     strategy:
       matrix:
-          python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "pypy-3.7", "pypy-3.8"]
+          python-version: ["3.7", "3.8", "3.9", "3.10", "pypy-3.8", "pypy-3.9"]
           twisted-version: ["lowest", "latest"]
           experimental: [false]
 
@@ -97,13 +97,12 @@ jobs:
       shell: python
       run: |
         table = {
-          "3.6": "py36",
           "3.7": "py37",
           "3.8": "py38",
           "3.9": "py39",
           "3.10": "py310",
-          "pypy-3.7": "pypy3",
           "pypy-3.8": "pypy3",
+          "pypy-3.9": "pypy3",
         }
         factor = table["${{ matrix.python-version }}"]
         print("::set-output name=value::" + factor)
diff --git a/.readthedocs.yml b/.readthedocs.yml
index f7e6d592..831e70b4 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -8,8 +8,12 @@ formats:
   - pdf
   - epub
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
 python:
-  version: 3.8
   install:
     - method: pip
       path: .
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 6bac9978..462ac279 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -1,7 +1,7 @@
 Developing
 ==========
 
-This project uses `Tox <https://tox.readthedocs.io/en/latest/config.html>`_ to manage virtual environments.
+This project uses `Tox 3 <https://tox.readthedocs.io/en/latest/config.html>`_ to manage virtual environments.
 
 To run the tests::
 
diff --git a/changelog.d/363.removal.rst b/changelog.d/363.removal.rst
new file mode 100644
index 00000000..6def8146
--- /dev/null
+++ b/changelog.d/363.removal.rst
@@ -0,0 +1 @@
+Support for Python 3.6, which has reached end of support, has been dropped.
diff --git a/changelog.d/365.feature.rst b/changelog.d/365.feature.rst
new file mode 100644
index 00000000..10527d4c
--- /dev/null
+++ b/changelog.d/365.feature.rst
@@ -0,0 +1 @@
+PyPy 3.9 is now supported.
diff --git a/changelog.d/365.removal.rst b/changelog.d/365.removal.rst
new file mode 100644
index 00000000..643891cf
--- /dev/null
+++ b/changelog.d/365.removal.rst
@@ -0,0 +1 @@
+Support for PyPy 3.7, which has reached end of support, has been removed.
diff --git a/setup.py b/setup.py
index b154265c..83027633 100644
--- a/setup.py
+++ b/setup.py
@@ -43,7 +43,7 @@
                 "werkzeug==2.0.3",
             ],
             "docs": [
-                "sphinx>=1.4.8",
+                "sphinx<7.0.0",  # Removal of 'style' key breaks RTD.
             ],
         },
         package_data={"treq": ["_version"]},
diff --git a/src/treq/_agentspy.py b/src/treq/_agentspy.py
index 42475bab..c7ad7f66 100644
--- a/src/treq/_agentspy.py
+++ b/src/treq/_agentspy.py
@@ -1,11 +1,11 @@
 # Copyright (c) The treq Authors.
 # See LICENSE for details.
-from typing import Callable, List, Optional, Tuple
+from typing import Callable, List, Optional, Tuple  # noqa
 
 import attr
 from twisted.internet.defer import Deferred
 from twisted.web.http_headers import Headers
-from twisted.web.iweb import IAgent, IBodyProducer, IResponse
+from twisted.web.iweb import IAgent, IBodyProducer, IResponse  # noqa
 from zope.interface import implementer
 
 
diff --git a/src/treq/auth.py b/src/treq/auth.py
index 6ed986dd..3a778cea 100644
--- a/src/treq/auth.py
+++ b/src/treq/auth.py
@@ -3,7 +3,7 @@
 from __future__ import absolute_import, division, print_function
 
 import binascii
-from typing import Union
+from typing import Union  # noqa
 
 from twisted.web.http_headers import Headers
 from twisted.web.iweb import IAgent
diff --git a/tox.ini b/tox.ini
index e1088df3..2c308623 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,8 +1,8 @@
 [tox]
 envlist =
-    {py36,py37}-twisted_lowest,
-    {pypy3,py36,py37,py38,py39,py310}-twisted_latest,
-    {pypy3,py36,py37,py38,py39,py310}-twisted_trunk,
+    py37-twisted_lowest,
+    {pypy3,py37,py38,py39,py310}-twisted_latest,
+    {pypy3,py37,py38,py39,py310}-twisted_trunk,
     twine, check-manifest, flake8, docs, coverage-report
 isolated_build = true
 
@@ -53,11 +53,12 @@ commands =
 [testenv:docs]
 extras = docs
 changedir = docs
+basepython = python3.8
 commands =
     sphinx-build -b html . html
 
 [testenv:coverage-report]
-depends = pypy3,py36,py37,py38,py39
+depends = pypy3,py37,py38,py39
 commands =
     coverage combine
     coverage report