Don't use deprecated setup.py build_sphinx

doc/conf.py

@@ -148,9 +148,9 @@ def linkify_issues_in_changelog(app, docname, source):
         # this path trickery is needed because this script can
         # be invoked with different working directories:
         # * running make in docs/
-        # * running python setup.py build_sphinx in the repo root dir
+        # * running tox -e docs in the repo root dir
 
-        with open(changelog_path) as f:
+        with open(changelog_path, encoding="utf-8") as f:
             changelog = f.read()
 
         def linkify(match):

setup.cfg

@@ -9,9 +9,6 @@ tag_date = true
 release = egg_info -Db ''
 upload = upload --sign --identity=36580288
 
-[build_sphinx]
-warning-is-error = 1
-
 [extract_messages]
 mapping_file = babel.cfg
 output_file = sphinx/locale/sphinx.pot

tox.ini

@@ -83,7 +83,7 @@ description =
 extras =
     docs
 commands =
-    python setup.py build_sphinx {posargs}
+    python -m sphinx -M html ./doc ./build/sphinx -W
 
 [testenv:docslint]
 basepython = python3