py3: add __pycache__ to .hgignore

Something changed with Python 3 just like million on things. Maybe they should
named Python 3 as anaconda or cobra instead of just increasing the version
number.

This makes test-custom-filters.t pass on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D5039

contrib/python3-whitelist

@@ -116,6 +116,7 b' test-convert-tagsbranch-topology.t'
 test-copy-move-merge.t
 test-copy.t
 test-copytrace-heuristics.t
+test-custom-filters.t
 test-debugbuilddag.t
 test-debugbundle.t
 test-debugcommands.t

tests/test-custom-filters.t

@@ -27,6 +27,7 b''
   > .hgignore
   > prefix.py
   > prefix.pyc
+  > __pycache__/
   > EOF
 
   $ cat > stuff.txt <<EOF