ENH: Add a mypy plugin for exposing platform-specific extended-precision types

[BvB93]

This PR modifies numpy's mypy plugin such that it manages all platform-specific extended-precision types (e.g. float128). More specifically, it removes all extended-precision types unavailable to the platform in question, rather than adding them.

The advantage of their removal (rather than addition) is that aforementioned types will still be available for use even without enabling the plugin. On the flip side, this does mean that, without the plugin, every single extended-precision type is "available" on every single platform.

The plugin can be enabled via one's mypy.ini file:

[mypy]
plugins = numpy.typing.mypy_plugin

Examples

With the plugin enabled.

from typing import TYPE_CHECKING
import numpy as np

if TYPE_CHECKING:
    # Depending on the platform:
    # note: Revealed type is 'numpy.floating[numpy.typing._128Bit*]'
    # or 
    # error: Module has no attribute "float128"; maybe "float64", "float32", or "float16"?
    reveal_type(np.float128())

[BvB93]

Effectively the plugin removes all import statements that are invalid for the platform in question.

The full list of (potential) extended-precision types was taken from numpy/__init__.pxd.

[BvB93]

Anyone knows what's going with the PyPy tests?
The failure seems to be unrelated, but it's failing nevertheless.

[mattip]

Anyone knows what's going with the PyPy tests?

It uses a nightly build since there is not an official release for win64 yet. Let's see if this repeats.

[charris]

Thanks Bas.