support defining callable get_version kwargs in in pyproject.toml tool.setuptools_scm

Currently when using the non-legacy pyproject.toml setuptools_scm configuration, you are unable to define inline callable version schemes - you’re forced to publish a package on pypi and use its entry point. setuptools_scm should provide a declarative alternative to:

#setup.py
import setuptools

def myversion():
    from setuptools_scm.version import get_local_dirty_tag
    def clean_scheme(version):
        return get_local_dirty_tag(version) if version.dirty else '+clean'

    return {'local_scheme': clean_scheme}

setup(
    ...,
    use_scm_version=myversion,
    ...
)

for example:

# pyproject.toml

[tool.setuptools_scm]
local_scheme = "scm_schemes:clean_scheme"

# scm_schemes.py
from setuptools_scm.version import get_local_dirty_tag

def clean_scheme(version):
    return get_local_dirty_tag(version) if version.dirty else '+clean'

[RonnyPfannschmidt]

@ahopkins things changed a bit as more and more static metadata stuff is incoming

im still not convinced its a good idea to support random imports for the schmes, but im happy to accept a pr

[RonnyPfannschmidt]

in the setuptools world its perfectly valid to just use setup.py for that

so im treating this one as low prio, but im happy to review a contribution