Allowing Generic Atom

[MatthieuDartiailh]

With the improved support for typing in atom, I am wondering if it would make to have some special support for the following:

Currently this will work and will statically type check:

from typing import Generic, TypeVar
from atom.api import Atom, List

T = TypeVar("T", bound=object)


class A(Atom, Generic[T]):

    # Mypy dislike the use of T outside an annotation 
    # but with #133 we could just use an annotation
    l: List[T] = List(T)  # type: ignore


class B(A[int]):
    pass


b = B()
b.l.append("")  # mypy complain atom is happy though

The fact that atom does not complain at runtime is something I find sad (I work currently in a context where I appreciate the strictest possible typing). I believe we could alter AtomMeta to make it so that the member becomes specialized to also do validation at runtime. Would that be of interest to you @clamdad @frmdstryr ?

One thing I am undecided upon yet if we go this is if A[int] should create a subclass or should only its use in B creation trigger specialization.

[frmdstryr]

I'd prefer if A[int] made it's own singleton type otherwise I don't think isinstance would work?

B = A[int]
C = A[int]
assert B is C 

class D(A[int]):
  pass
assert isinstance(D, B)