Use legacy mode for editable install, better supported by mypy

Author: bluebird75

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
       run: |
         python -m pip install pip==21.2.4 # last version that works for editable install + mypy
         python -m pip install pyside6==${{ env.PYSIDE6_VERSION }} pytest mypy==${{ matrix.mypy-version }}
-        pip install -e .
+        pip install -e . --config-settings editable_mode=compat
         pip list
 
     - name: run tests