pytype: work around wrong ImportError flagging
As documented in https://github.com/google/pytype/issues/163, newer versions
of Pytype do not understand caught `ImportError`, so we temporarily ignore
them where applicable.
#if no-windows
$ 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 2>/dev/null
foobar
#endif