setup.py: set version to 1.20.0.

Author: julian-smith-artifex-com

setup.py

@@ -673,7 +673,7 @@ def get_mupdf():
 
 setup(
     name="PyMuPDF",
-    version="1.20.0rc3",
+    version="1.20.0",
     description="Python bindings for the PDF toolkit and renderer MuPDF",
     long_description=readme,
     long_description_content_type="text/markdown",