Pin tox < 4.0.0

The most recent tox release, 4.x.y, 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 (likely because it's using wheel builds instead of
sdists). 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
the new version tox.

.github/workflows/main.yml

@@ -28,7 +28,7 @@ jobs:
             ${{ runner.os }}-${{ matrix.python-version }}-pip-
             ${{ runner.os }}-${{ matrix.python-version }}
       - name: Install Deps
-        run: python -m pip install -U tox setuptools virtualenv wheel
+        run: python -m pip install -U 'tox<4' setuptools virtualenv wheel
       - name: Install and Run Tests
         run: tox -e py
         if: runner.os != 'macOS'
@@ -57,7 +57,7 @@ jobs:
             ${{ runner.os }}-${{ matrix.python-version }}-pip-
             ${{ runner.os }}-${{ matrix.python-version }}-
       - name: Install Deps
-        run: python -m pip install -U tox
+        run: python -m pip install -U 'tox<4'
       - name: Run lint
         run: tox -epep8
   cover:
@@ -79,7 +79,7 @@ jobs:
             ${{ runner.os }}-${{ matrix.python-version }}-pip-
             ${{ runner.os }}-${{ matrix.python-version }}-
       - name: Install Deps
-        run: python -m pip install -U tox
+        run: python -m pip install -U 'tox<4'
       - name: Run coverxml
         run: tox -ecoverxml
       - name: codecov
@@ -111,7 +111,7 @@ jobs:
             ${{ runner.os }}-pip-
             ${{ runner.os }}-
       - name: Install Deps
-        run: python -m pip install -U tox
+        run: python -m pip install -U 'tox<4'
       - name: Build Docs
         run: tox -edocs
       - uses: actions/upload-artifact@v2