fix for CanvasCache._deps memory leak (found by John Goodfellow)
fix for CanvasCache._deps memory leak (found by John Goodfellow)

--HG--
extra : convert_revision : a80ebfc6c32ab1fe51a59e7e3ba15f264a4c7f62

hkoof/urwid,urwid/urwid,westurner/urwid,foreni-packages/urwid,mountainstorm/urwid,ivanov/urwid,zyga/urwid,harlowja/urwid,drestebon/urwid,westurner/urwid,Julian/urwid,foreni-packages/urwid,Julian/urwid,mountainstorm/urwid,hkoof/urwid,mountainstorm/urwid,inducer/urwid,bk2204/urwid,inducer/urwid,douglas-larocca/urwid,drestebon/urwid,rndusr/urwid,rndusr/urwid,drestebon/urwid,Julian/urwid,foreni-packages/urwid,inducer/urwid,wardi/urwid,hkoof/urwid,tonycpsu/urwid,westurner/urwid,urwid/urwid,urwid/urwid,douglas-larocca/urwid,douglas-larocca/urwid,rndusr/urwid,ivanov/urwid,zyga/urwid,bk2204/urwid,tonycpsu/urwid,harlowja/urwid,wardi/urwid,harlowja/urwid,ivanov/urwid,tonycpsu/urwid,bk2204/urwid,zyga/urwid,wardi/urwid