Type stub

[thisiscam]

An initial attempt that addresses #68.

The current PR provides the following working functionalities:

• automatically generates a type stub
• installs the type stub through setup.py

Missing:

• the types for callbacks are not accurate enough -- I think that's worth another PR.
• typing for helpers in islpy/__init__.py -- this will be in a separate PR.
• make this more useful for documentation (per @inducer's comment in Type stubs for Python typing #68) -- IMO, this type stub could be useful for the time being, and it also seems to be quite standard among libraries with native extensions (e.g. pytorch)

This PR also depends on #69 . We should probably resolve that one first.

[inducer]

This seems to fail most of the CI... request review when you think it's ready.

[thisiscam]

Fixed the style issues. Please give it a look?