Explicitly trigger GC prior to counting; fixes #39.

Thanks to a touch of assistance from #pypy, an explicit call to `gc.collect()` will force the objects to be collected rather than letting the runtime defer cleanup to some indeterminate future time.

**Huge warning:** This is *not* guaranteed to continue working in future pypy versions.  Attempting to manipulate the GC is generally an Unwise Thing to Do™, Not Portable Between Runtimes™, and not too dissimilar to direct manipulation of the GIL.  It'll probably be fine, but pypy supports modular GC policies and this has only been tested under the default policy.