Add type hints and ruff/mypy config

Annotate all functions and check them with mypy --strict (needs
python-pygobject-stubs). pyproject.toml holds only tool config since
the PKGBUILD installs the script directly.

pyproject.toml

@@ -0,0 +1,10 @@
+[tool.ruff]
+line-length = 120
+extend-include = ["xautolock-inhibit"]
+
+[tool.ruff.lint]
+select = ["E", "F", "W", "I", "B", "UP"]
+
+[tool.mypy]
+files = ["xautolock-inhibit"]
+strict = true