Update the SOURCE_URI in pyspecific.py to point to the new branch (GH-25977)

Author: pablogsal

Doc/tools/extensions/pyspecific.py

@@ -44,7 +44,7 @@
 
 
 ISSUE_URI = 'https://bugs.python.org/issue%s'
-SOURCE_URI = 'https://github.com/python/cpython/tree/master/%s'
+SOURCE_URI = 'https://github.com/python/cpython/tree/main/%s'
 
 # monkey-patch reST parser to disable alphabetic and roman enumerated lists
 from docutils.parsers.rst.states import Body