PyAPI: move bpy_types.py to a private module

Use an underscore prefix as this module should not be accessed directly.

doc/python_api/sphinx_doc_gen.py

@@ -791,7 +791,7 @@ def pyfunc2sphinx(ident, fw, module_name, type_name, identifier, py_func, is_cla
         # would be listed in documentation which isn't useful.
         #
         # However, excluding all of them is also incorrect as it means class methods defined
-        # in `bpy_types.py` for example are excluded, making some utility functions entirely hidden.
+        # in `_bpy_types.py` for example are excluded, making some utility functions entirely hidden.
         if (bl_rna := getattr(py_func.__self__, "bl_rna", None)) is not None:
             if bl_rna.functions.get(identifier) is not None:
                 return