CLEAN: Align Groupby types

[Dr-Irv]

• Closes CLEAN: Fix up Groupby classes #149
• Tests added: Please use assert_type() to assert the type of any return value
  • None

pandas has BaseGroupBy (which should be private), whereas the stubs had _GroupBy, but I wanted to get things aligned. Probably BaseGroupBy should be private in pandas.

In addition, SelectionMixin is really an abstract class, so removing aggregate makes sense, as the pandas implementation raises an exception anyway.

[twoertwein]

In addition, SelectionMixin is really an abstract class, so removing aggregate makes sense, as the pandas implementation raises an exception anyway.

Could let it return NoReturn but that is not worth it.

[twoertwein]

Thank you @Dr-Irv