ENH: Add annotations to 3 functions in np.core.function_base

[BvB93]

This pull requests adds annotations to 3 functions in np.core.function_base:

• linspace
• logspace
• geomspace

As the main __init__.pyi file in numpy is starting to get somewhat cluttered
the respective stubs have been defined for the relevant sub-module instead.

Secondly, the docstring of logspace has been updated in order to clarify that
its base parameter can be array_like; not just a float.

[BvB93]

A bit of context about the back-and-forth in 72e11d3, c9c44fd & 68832b3:

Typing-extensions 3.7.4.3 added a backport of the (in python 3.8 introduced) SupportsIndex protocol, but the necessary stub files are still missing from typeshed (python/typeshed#4442). So for the time being we'll have to define it ourselves.

I suspect that this minor inconvience will be fixed somewhere around the release of either mypy 0.783 or 0.784.

[mattip]

There are a number of PRs with the static typing tag (including this one) that seem ready. Who besides @BvB93 is reviewing these?

[rgommers]

There are a number of PRs with the static typing tag (including this one) that seem ready. Who besides @BvB93 is reviewing these?

I'm trying to when I can, and I've merged some - but I'm pretty low on bandwidth.

[charris]

@BvB93 Needs rebase.

[charris]

Thanks @BvB93 .