Remove __pycache__ directories before release. (#182)

doc/howto_release.md

@@ -77,6 +77,7 @@ find . -name '*.rej'  | xargs rm
 find . -name '*.o'    | xargs rm
 find . -name '*.pyc'  | xargs rm
 find . -name 'OBJ.*'  | xargs rm -r
+find . -name '__pycache__' | xargs rm -r
 rm -f gui/wxpython/menustrings.py gui/wxpython/build_ext.pyc gui/wxpython/xml/menudata.xml gui/wxpython/xml/module_tree_menudata.xml
 chmod -R a+r *
 ```