Try to make GitHub Actions automatically use appropriate Tox envs by using

`tox-gh-actions` package.

.github/workflows/ci.yml

@@ -40,17 +40,15 @@ jobs:
 
       - name: Set up Python ${{ matrix.python-version }}
         if: ${{ matrix.os != 'windows-latest' }}
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
 
       # See note below about need for using 32-bit Python 2.7 on
-      # Windows.  The extra handling here for Python 3.6 on Windows is
-      # because I could not figure out the right GitHub Actions
-      # expression to do this in a better way.
+      # Windows.
       - name: Set up Python ${{ matrix.python-version }} [Windows x64]
-        if: ${{ ( matrix.os == 'windows-latest' ) && ( matrix.python-version == '3.6' ) }}
-        uses: actions/setup-python@v1
+        if: ${{ ( matrix.os == 'windows-latest' ) && ( matrix.python-version != '2.7' ) }}
+        uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
           architecture: 'x64'
@@ -89,14 +87,14 @@ jobs:
 
       - name: Install Python packages
         run: |
-          pip install --upgrade codecov tox setuptools
+          pip install --upgrade codecov tox tox-gh-actions setuptools
           pip list
 
       - name: Display tool versions
         run: python misc/build_helpers/show-tool-versions.py
 
-      - name: Run "tox -e py27-coverage"
-        run: tox -e py27-coverage
+      - name: Run tox for corresponding Python version
+        run: python -m tox
 
       - name: Upload eliot.log in case of failure
         uses: actions/upload-artifact@v1

tox.ini

@@ -3,11 +3,21 @@
 # test suite on all supported python versions. To use it, "pip install tox"
 # and then run "tox" from this directory.
 
+# Map Python versions in GitHub Actions to tox environments to run.
+[gh-actions]
+python =
+    2.7: py27-coverage,codechecks
+    3.6: py36-coverage
+    3.7: py37-coverage
+    3.8: py38-coverage
+    3.9: py39-coverage,typechecks
+    pypy-3.7: pypy3
+
 [pytest]
 twisted = 1
 
 [tox]
-envlist = typechecks,codechecks,py27,py36,pypy27
+envlist = typechecks,codechecks,py27,py36,py37,py38,py39,pypy27,pypy3
 minversion = 2.4
 
 [testenv]