Allow specifying a function that gets the version

[flying-sheep]

People like using versioneer, setuptools_scm, or get_version to retrieve the version from other places than the main module’s code.

Allowing to specify a function would have the advantage that people don’t have to make sure the main module is importable without all dependencies being installed.

It would neatly fix things like #253 (without ignoring errors) and #257 (without adding complexity to flit)

[astrojuanlu]

I tried using versioneer in a flit project and, by "tricking it" a bit (i.e. creating an empty setup.py and then removing it, and removing also the MANIFEST.in) I got it to work. So I don't see what changes should flit make.

[flying-sheep]

Ah, yes. Versioneer will work, since it works by copying code into your project instead of being a dependency. But if you use something that works as a dependency, #253 will prevent stuff from working:

from setuptools_scm import get_version
__version__ = get_version(root='..', relative_to=__file__)