Refactor mypy_plugin.py for Improved Code Quality and Compliance

[marcfreir]

Hello, Markus Siemens & TinyDB team!

The TinyDB is a very interesting project. The project is well done. Upon reviewing the mypy_plugin.py file, it has a Pylint rating of 2.00/10, indicating multiple areas for improvement. This is just to be more formal with the evaluation. I made a few analyses, and just as a minor improvement, I could suggest, in the hook function inside get_dynamic_class_hook contains type-checking logic that could be error-prone and difficult to maintain. My suggestion is Refactor hook as a separate method, which can make the main logic clearer and easier to debug. For example:

     def _hook_logic(ctx: DynamicClassDefContext):
         klass = ctx.call.args[0]
         if not isinstance(klass, NameExpr):
             raise TypeError("Expected a NameExpr instance.")
         
         type_name = klass.fullname
         if type_name is None:
             raise ValueError("Type name cannot be None.")
         
         qualified = self.lookup_fully_qualified(type_name)
         if qualified is None:
             raise LookupError(f"Failed to find a fully qualified name for {type_name}")
         
         ctx.api.add_symbol_table_node(ctx.name, qualified)
     
     def get_dynamic_class_hook(self, fullname: str) -> CB[DynamicClassDef]:
         if fullname == 'tinydb.utils.with_typehint':
             return self._hook_logic
         return None

And thanks for the awesome project!