CI: install dependencies when running mypy

Author: dnicolodi

.github/workflows/tests.yml

@@ -348,6 +348,9 @@ jobs:
         with:
           python-version: 3.9
 
+      - name: Install dependencies
+        run: python -m pip install .
+
       - name: Install mypy
         run: python -m pip --disable-pip-version-check install mypy==1.5.1