widgets.spellcheck: use older PyQt4 API

We (still) support older versions of PyQt4 so avoid pyqtSignal for now.
  git-cola 2.0 can do the pyside thing.

Signed-off-by: David Aguilar <davvid@gmail.com>
git-cola v1.8.3

Signed-off-by: David Aguilar <davvid@gmail.com>