Add internal classes and add type hints + mypy.

setup.cfg

@@ -1,6 +1,9 @@
 [flake8]
 max-line-length = 88
 
+[ruff]
+line-length = 88
+
 [isort]
 multi_line_output = 3
 include_trailing_comma = True
@@ -8,3 +11,8 @@ force_grid_wrap = 0
 use_parentheses = True
 ensure_newline_before_comments = True
 line_length = 88
+add_imports =
+    from __future__ import annotations
+
+[mypy]
+ignore_missing_imports = True