Complete transition of setup and versioning from setup.py to pyproject.toml

[jeffjennings]

See #36 (comment) :

          `version.py` should be generated by `setuptools_scm` during the build process, so that file should be there by the time an import happens. But I think it's not configured correctly -- try adding:

[project]
dynamic = ["version"]

[tool.setuptools_scm]
version_file = "astropylibrarian/_version.py"

to the pyproject.toml file. Then, the code in __init__.py can be what @pllim suggested:

try:
    from ._version import version as __version__
except ImportError:
    __version__ = ""

Originally posted by @adrn in #36 (comment)