Add typing_extensions as a dependency

[person142]

The typing_extensions module contains backports of new typing features for earlier versions of Python. It is understood by type checkers like Mypy and Pyre. In #59 it is proposed that we add it as a dependency, which I think is a good idea. Beyond just that PR, some potential benefits are:

• Using Literal to better type specific (but fairly common) overloads, e.g.

  np.array(..., dtype="float")

• Protocol might be a good way to approach typing ArrayLike
• Using TypedDict to better describe complex dtypes.

I imagine that the biggest concern would be forcing it to be a hard dependency of NumPy should the stubs be moved there, but I believe we can avoid that.

• If everything stays stubs only, then it's not a problem

• If we do add annotations to source files, we can still work around it with constructs like

  from typing import TYPE_CHECKING
  
  if TYPE_CHECKING:
      from typing_extensions import Literal
  
  x: 'Literal[1]' = 1  # Or use `from __future__ import annotations` post 3.8
  ...

Objections? In the absence of them we'll move forward with including it.

[rgommers]

Sounds like a good idea to me

[BvB93]

A separate pull request has just been created for the addition of typing_extensions (#63).

[person142]

Thanks. I’ll give people a little bit longer to raise any objections here, and then we’ll move forward.

[person142]

Closed by #63.