[PATCH 1 of 2 v2] py3: conditionalize the raise statement

[Pulkit Goyal]

> This change for mercurial/pure/mpatch.py doesn't related to fixing up
> in this patch. This is already fixed by my previous patch below:

Yeah thats why I have mentioned in the commit message that this patch
also contains an update
 to test-check-py3-compat.t . I saw your patch being pushed, if people
are happy to push my patch,
I will send an updated version.