py3: use `except error as e` instead of `except error, e`

The latter throws SyntaxError on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D3613

tests/test-parseindex.t

@@ -184,7 +184,7 b' Test corrupted p1/p2 fields that could c'
   >     try:
   >         f()
   >         print('uncaught buffer overflow?')
-  >     except ValueError, inst:
+  >     except ValueError as inst:
   >         print(inst)
   > EOF