Use tox-py in CI

This is a [tox plugin I just made](https://pypi.org/project/tox-py/) to replace this shell-fu.

.github/workflows/tests.yml

@@ -20,14 +20,16 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
+
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
+
       - name: Install dependencies
         run: |
-          python -m pip install --upgrade pip wheel setuptools tox
+          python -m pip install --upgrade pip setuptools wheel
+          python -m pip install --upgrade tox tox-py
+
       - name: Run tox targets for ${{ matrix.python-version }}
-        run: |
-          ENV_PREFIX=$(tr -C -d "0-9" <<< "${{ matrix.python-version }}")
-          TOXENV=$(tox --listenvs | grep "^py$ENV_PREFIX" | tr '\n' ',') python -m tox
+        run: tox --py current