setup.py: force creation of new PyMuPDFb wheels.

The layout of PyMuPDFb has changed with the move to rebased stage 2.

Author: julian-smith-artifex-com

setup.py

@@ -1073,7 +1073,7 @@ def sdist():
 #
 
 version = '1.23.9rc1'
-version_b = '1.23.7'
+version_b = '1.23.9rc1'
 
 tag_python = None
 requires_dist = None,