Added support to create Generic models

[rschyboll]

Description

Removed getitem from ModelMeta and replaced it with _class_getitem in Model class, which is described in PEP 560.
Also added to the function one condition, which checks if the key passed in is a tuple with classes or a single class, if it is, then it returns a _GenericAlias, which allows python to create that model as a Generic class instance.
If the user passes a key, which is not a class, it behaves the same way as before.

Motivation and Context

I was trying to make generic Models, which would allow me to create some generic methods, that would allow me to use them in all of my models. One example of such method would be:

_VIEW = TypeVar("_VIEW", bound="BaseView")
_MODEL = TypeVar("_MODEL", bound="BaseModel[_VIEW, _CREATE]")  # type: ignore


class BaseModel(Model, Generic[_CREATE]):
    def to_view(self) -> _VIEW:
        raise NotImplementedError()

    @overload
    @classmethod
    async def get_all(cls: Type[_MODEL], view: Literal[True] = ...) -> List[_VIEW]:
        pass

It is a helper method, which returns a view class, that is later returned to the user, thanks to the Generic ability of that model, i need only to reimplement the abstract to_view method, and all the other helper methods have already the required type.

How Has This Been Tested?

Obviously i tested it only in my own code, so i couldn't cover all scenarios, but i also ran all tests from "make test", and "test_sqlite" without any errors.
In my understanding, it shouldn't affect any other areas, because the method before was only to get one model instance by using the models class as an array, accesing it by it's primary key. Hopefully i'm right, if not, then sorry in advance :(

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.