Historically, our get_package_version() method has returned

alpha/beta versions as, say, 3.0.21alpha1 or 3.0.21beta1. However,

Python packages use a instead of alpha and b instead of beta,

which leads to a warning every time we build a package. Worse, that

difference means we can't reliably call this function to get a suitable

verison string for matching a package filename.
This change updates this method to use the shortened a and b tags

instead.

Built a package and saw the correct filename, without warnings.
Used this in an upcoming change to correctly match a generated package

filename.