Add tests with minimum supported Pytest version

I've edited the Github workflows configuration to run tests with
the lowest version of pytest marked as supported. If anything in
a future commit breaks compatibility with those versions, we'll be
alerted by a test failure and we can adjust the version compatibility
accordingly.

Author: diazona

.github/workflows/tests.yml

@@ -17,6 +17,15 @@ jobs:
         # https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
         python-version: [3.5, 3.6, 3.7, 3.8, 3.9, '3.10', '3.11.0-beta - 3.11', 'pypy-3.7', 'pypy-3.8', 'pypy-3.9']
         extra: ['', '-smtp']
+        # The forced pytest versions correspond with the lower bounds in tox.ini
+        pytest-version: ['', '--force-dep pytest==4', '--force-dep pytest==6.2.4']
+        exclude:
+          - python-version: '3.5'
+            pytest-version: '--force-dep pytest==6.2.4'
+          - python-version: '3.10'
+            pytest-version: '--force-dep pytest==4'
+          - python-version: '3.11.0-beta - 3.11'
+            pytest-version: '--force-dep pytest==4'
       fail-fast: false
 
     steps:
@@ -30,4 +39,4 @@ jobs:
         python -m pip install --upgrade pip
         pip install tox
     - name: Test with tox
-      run: tox -vv -e py${{ matrix.extra }}
+      run: tox -vv -e py${{ matrix.extra }} ${{ matrix.pytest-version }}

tox.ini

@@ -19,7 +19,9 @@ deps =
     # tox's --force-dep option to find the earliest versions of pytest that are
     # known to work. If you add code that breaks with an old version of pytest,
     # feel free to raise these bounds to represent the earliest version that
-    # works with your code.
+    # works with your code. Don't forget to modify the corresponding entries in
+    # the Github workflows configuration file if you do change these lower
+    # bounds.
     pytest >=4, <8; python_version<'3.10'
     pytest >=6.2.4, <8; python_version>='3.10'
     pytest-cov