py3: add pycompat.open and replace open() calls
open() requires mode argument as unicodes on Python 3. This patch introduces
pycompat.open() which is inserted to files using transformer and replaces
builtins.open() calls.
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username
foobar
#endif