Test SETUPTOOLS_USE_DISTUTILS = stdlib, local on GH Actions

.github/workflows/main.yml

@@ -6,6 +6,9 @@ jobs:
   test:
     strategy:
       matrix:
+        distutils:
+        - stdlib
+        - local
         python:
         - pypy3
         - 3.6
@@ -15,7 +18,10 @@ jobs:
         - ubuntu-latest
         - macos-latest
         - windows-latest
+      fail-fast: false
     runs-on: ${{ matrix.platform }}
+    env:
+      SETUPTOOLS_USE_DISTUTILS: ${{ matrix.distutils }}
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python

tox.ini

@@ -7,14 +7,20 @@ toxworkdir={env:TOX_WORK_DIR:.tox}
 
 [testenv]
 deps =
+	pytest
 	# workaround for sphinx-doc/sphinx#9562
 	# TODO: remove after Sphinx>4.1.2 is available.
 	sphinx@git+https://github.com/sphinx-doc/sphinx; python_version>="3.10"
 commands =
-	pytest {posargs}
+	# We invoke pytest like this to avoid the warning:
+	#   UserWarning: Distutils was imported before Setuptools,
+	#   but importing Setuptools also replaces the `distutils`
+	#   module in `sys.modules`.
+	pytest -c 'import setuptools; import pytest; pytest.main("{posargs}".split())'
 usedevelop = True
 extras = testing
 passenv =
+	SETUPTOOLS_USE_DISTUTILS
 	windir  # required for test_pkg_resources
 
 [testenv:docs]