Set version for 0.3.0.beta1 release

[benjeffery]

No description provided.

[jeromekelleher]

For PEP 440 version numbering this should be 0.3.0b1 rather than 0.3.0.b1. Setuptools will normalise to 0.3.0b1, so that there's a slight difference between the package version and the __version__ value:

>>> import tskit
>>> tskit.__version__
'0.3.0.b1'
>>> 

and the package is tskit-0.3.0b1.tar.gz (this is also the version string from pypi).

I don't think it'll make any difference for the beta though.

[benjeffery]

I realised this at the time, but went with the convention specified in the comment in _version.py as setuptools was still happy.

[jeromekelleher]

We should update the comment there to avoid confusion I think - 0.3.0.devN is correct for development releases but pre-releases have a different convention. 🤷