Reduce test matrix to just the oldest and newest Python versions

Testing the versions in between doesn't really bring much benefit, and
it becomes impractical when the range of supported versions grows.

.github/workflows/ci.yml

@@ -10,12 +10,6 @@ jobs:
       - uses: actions/setup-python@v1
         with:
           python-version: "3.6"
-      - uses: actions/setup-python@v1
-        with:
-          python-version: "3.7"
-      - uses: actions/setup-python@v1
-        with:
-          python-version: "3.8"
       - uses: actions/setup-python@v1
         with:
           python-version: "3.9"

tox.ini

@@ -1,6 +1,7 @@
 [tox]
-# When adding a new Python version here, please also update the list of Python versions called by the GitHub Actions workflow (.github/workflows/ci.yml).
-envlist = py{36,37,38,39},flake8,mypy,package
+# When updating the Python versions here,
+# please also update the corresponding Python versions in the GitHub Actions workflow (.github/workflows/ci.yml).
+envlist = py{36,39},flake8,mypy,package
 
 [testenv]
 commands = python -m unittest discover --start-directory ./tests