Enhance Bzr.version_cmp to handle non-numeric versions

Enhance Bzr.version_cmp to handle non-numeric versions

bzr uses non-numeric tags to indicate prereleases; previously, this
triggers an exception in be's Bzr module as version comparison is only
supported between version strings that only contain numbers and dots.

This patch extends version_cmp to support a single non-numeric
pre-release string of arbitrary length (e.g. 'a', 'b', 'pre', 'rc'), and
extends the docstring tests to cover this extension.

Signed-off-by: Michel Alexandre Salim <salimma@fedoraproject.org>