Tell mypy to assume Python 3.9

This should help catch incompatible code faster.

Author: yut23

.github/workflows/lint.yml

@@ -62,7 +62,7 @@ jobs:
         python -m pip install --upgrade pip
         python -m pip install ".[lint]"
     - name: Type check with mypy
-      run: mypy --warn-redundant-casts --warn-unused-ignores breathe tests
+      run: mypy --warn-redundant-casts --warn-unused-ignores --python-version 3.9 breathe tests
 
   twine:
     runs-on: ubuntu-latest

Makefile

@@ -44,4 +44,4 @@ black:
 
 .PHONY: type-check
 type-check:
-	mypy --warn-redundant-casts --warn-unused-ignores breathe tests
+	mypy --warn-redundant-casts --warn-unused-ignores --python-version 3.9 breathe tests