Pin tox version used in CI to < 4.0.0 (#761)

The most recent tox release, 4.0.0, is a major rewrite of the internals
of tox and several things behave quite differently. This new release
is causing CI jobs to fail as something in incompatible with our tox
configuration. This commit pins the tox version we're using in CI to
unblock things until we can update the tox configuration to be
compatible with both the new and old versions of tox.

.github/workflows/docs_dev.yml

@@ -17,7 +17,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -U virtualenv setuptools wheel tox
+        pip install -U virtualenv setuptools wheel 'tox<4'
         sudo apt-get install graphviz pandoc
     - name: Build and publish
       env:

.github/workflows/docs_release.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -U virtualenv setuptools wheel tox
+        pip install -U virtualenv setuptools wheel 'tox<4'
         sudo apt-get install graphviz pandoc
     - name: Build and publish
       env:

.github/workflows/main.yml

@@ -84,7 +84,7 @@ jobs:
           profile: minimal
           default: true
       - name: 'Install dependencies'
-        run: python -m pip install --upgrade tox
+        run: python -m pip install --upgrade 'tox<4'
       - name: 'Install binary dependencies'
         run: sudo apt-get install -y graphviz
         if: runner.os == 'Linux'
@@ -118,7 +118,7 @@ jobs:
           profile: minimal
           default: true
       - name: 'Install dependencies'
-        run: python -m pip install --upgrade tox
+        run: python -m pip install --upgrade 'tox<4'
       - name: 'Install binary dependencies'
         run: sudo apt-get install -y graphviz
         if: runner.os == 'Linux'
@@ -195,7 +195,7 @@ jobs:
       - name: Install binary deps
         run: sudo apt-get install -y graphviz
       - name: Install deps
-        run: pip install -U tox
+        run: pip install -U 'tox<4'
       - name: Build Docs
         run: tox -edocs
       - uses: actions/upload-artifact@v2