Allow numbers in releaselevel.

.bumpversion.cfg

@@ -2,7 +2,7 @@
 current_version = 4.1.0
 commit = True
 tag = True
-parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)(?P<releaselevel>[a-z]+)?
+parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)(?P<releaselevel>[a-z\d]+)?
 serialize =
 	{major}.{minor}.{patch}{releaselevel}
 	{major}.{minor}.{patch}