Use 3-part version numbers also for 3.0, i.e. 3.0.0, as we did for previous releases.

See https://github.com/cython/cython/issues/4022#issuecomment-870394893

Cython/Shadow.py

 # cython.* namespace for pure mode.
 from __future__ import absolute_import
 
-__version__ = "3.0a7"
+__version__ = "3.0.0a7"
 
 try:
     from __builtin__ import basestring