py3: don't use dict.iterkeys() in hgext/fastannotate/context.py
dict.iterkeys() is not present on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D4431
#require serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ $PYTHON "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py