Pin to tox<4

Pin to tox<4 to avoid breakage caused by tox 4.0.0 backwards-incompatibilities.

See hypothesis/test-pyapp#6 for details.

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
       run: python -m pip install --upgrade pip
 
     - name: Install tox
-      run: python -m pip install tox
+      run: python -m pip install 'tox<4'
 
     - name: Checkout git repo
       uses: actions/checkout@v2

bin/install-python

@@ -39,7 +39,7 @@ do
     # Install tox in this version of Python if it's not already installed.
     if ! "$(pyenv root)/versions/$python_version/bin/tox" --version > /dev/null 2>&1
     then
-        "$(pyenv root)/versions/$python_version/bin/pip" install --quiet --disable-pip-version-check tox > /dev/null
+        "$(pyenv root)/versions/$python_version/bin/pip" install --quiet --disable-pip-version-check 'tox<4' > /dev/null
         pyenv rehash
     fi
 done < .python-version

tox.ini

@@ -7,8 +7,8 @@ filterwarnings =
 [tox]
 envlist = tests
 skipsdist = true
-minversion = 3.8.0
 requires =
+    tox>=3.8.0,<4
     tox-faster
     tox-pyenv
     tox-run-command