Checks only platotk with mypy

Running mypy on the whole repo generates errors when two folders contain two
python files with the same name. This generates a mypy error and a simple
solution to it is not possible.
See related issues:
- python/mypy#10428
- python/mypy#4008

For this reason, we restrict type checking only on the main package code.

.github/workflows/mypy.yml

@@ -20,4 +20,4 @@ jobs:
         pip install -r requirements/dev_requirements.txt
     - name: Analysing code with mypy
       run: |
-        mypy --explicit-package-bases $(git ls-files '*.py')
+        mypy src/platotk