implement from __future__ import annotations

[dhalbert]

For type annotation purposes, we would like to write code that uses:

from __future__ import annotations

This from __future__ is part of PEP 563, which allows postponed evaluation of annotations, so that forward references work without stringifying the annotations. It is available in CPython 3.7 and up, and a lot of CPython .py code uses it. We would like to use it in code that can be used in both CPython and CircuitPython.

MicroPython (and CircuitPython) already effectively implement PEP 563 in a no-op way by totally ignoring all annotations. However, MicroPython does not provide a from __future__ import ... at all. This PR implements a no-op __future__ module with a single member annotations. In CPython, members of __future__ are _Feature objects, with version information. In this PR, annotations is just True.

This adds 100 bytes to the Trinket M0 build and presumably other builds. I think this is worth it to allow compatible code.

Note that we cannot do this workaround:

try:
    from __future__ import annotations
except ImportError:
    pass

because in CPython, a from __future__ must be absolutely the first statement in a source file. We don't check this restriction, and I don't think it's necessary.

[tannewt]

Thanks for this! One fix for the doc build.

[jepler]

Is fine by me, once the CI is content.

[dhalbert]

@jepler ready to re-review, thanks.

[jepler]

Thank you!

addressed

[dhalbert]

Consider backporting to 7.2.x.