pyproject.toml: Add tool.mypy section (MR 2464)

And remove --check-untyped-defs from mypy.sh script to avoid
duplication.

.ci/mypy.sh

@@ -11,4 +11,4 @@ fi
 set -x
 
 pip install --break-system-packages --no-warn-script-location mypy
-python -m mypy --color-output --check-untyped-defs pmbootstrap.py
+python -m mypy --color-output pmbootstrap.py

pyproject.toml

@@ -50,3 +50,5 @@ addopts = [
     "--basetemp=.pytest_tmp"
 ]
 
+[tool.mypy]
+check_untyped_defs = true