Python 3.2 workaround: Drop usage of clear()

The clear() collection method is not yet available in Python 3.2, but as long
as snakefruit still runs Ubuntu 12.04 we need to get along without it.

britney.py

@@ -2580,7 +2580,7 @@ class Britney(object):
                         self.output_write("  most: (%d) .. %s\n" % (len(selected), " ".join(x.uvname for x in selected[-20:])))
                     nuninst_last_accepted = nuninst_after
                     rescheduled_packages.extend(maybe_rescheduled_packages)
-                    maybe_rescheduled_packages.clear()
+                    maybe_rescheduled_packages = []
                 else:
                     broken = sorted(b for b in nuninst_after[failed_arch]
                                     if b not in nuninst_last_accepted[failed_arch])