Improve type-hints in function definitions

[neNasko1]

As there are current efforts to refactor spox and prepare it for a 1.0 release I have decided to look around the codebase for places where additional typing information can be provided. The changes are mainly around type-hinting in function declarations.

This PR centres around the following diff and fixing the type-errors discovered by it:

diff --git a/pyproject.toml b/pyproject.toml
index 8f5e249..ec2459f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -62,6 +62,7 @@ ignore_missing_imports = true
 no_implicit_optional = true
 check_untyped_defs = true
 warn_unused_ignores = true
+disallow_untyped_defs = true
 
 [tool.pytest.ini_options]
 # This will be pytest's future default.

type: ignore-s are only introduced in parts of the code that deals with the deprecated operator_overloading functionality.

[cbourjau]

Thanks! I have just two minor comments.

[cbourjau]

Thanks again! I have but one minor comment.