Run mypy type checking with the minimum supported Python version

.github/workflows/tests.yml

@@ -95,6 +95,10 @@ jobs:
       - name: Setup Poetry
         uses: matrix-org/setup-python-poetry@v1
         with:
+          # We set this as our minimum supported Python version. We use the oldest
+          # Python version because later Python versions can include some overloads
+          # which don't work in the older versions.
+          python-version: '3.8'
           # We want to make use of type hints in optional dependencies too.
           extras: all
           # We have seen odd mypy failures that were resolved when we started

changelog.d/15602.misc

@@ -0,0 +1 @@
+Run mypy type checking with the minimum supported Python version to catch new usage that isn't backwards-compatible.