dependencies: avoid Package-is-not-iterable in graph_reverse_depends doctest

dependencies: avoid Package-is-not-iterable in graph_reverse_depends doctest

Dependency is a subclass of Query, so we need to initialize it with a
CPV string, not a Package instance.  The old doctest was raising:

  File ".../pym/gentoolkit/dependencies.py", line 202, in
    gentoolkit.dependencies.Dependencies.graph_reverse_depends
  Failed example:
    deptree = ffmpeg.graph_reverse_depends(
        only_direct=False,  # Include indirect revdeps
        pkgset=installed)   # from installed pkgset
  Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python3.2/doctest.py", line 1288, in __run
        compileflags, 1), test.globs)
      File "<doctest gentoolkit.dependencies.Dependencies.graph_reverse_depends[5]>", line 3, in <module>
        pkgset=installed)   # from installed pkgset
      File ".../pym/gentoolkit/dependencies.py", line 237, in graph_reverse_depends
        pkgset = tuple(Dependencies(x) for x in pkgset)
      File ".../pym/gentoolkit/dependencies.py", line 237, in <genexpr>
        pkgset = tuple(Dependencies(x) for x in pkgset)
      File ".../pym/gentoolkit/dependencies.py", line 45, in __init__
        Query.__init__(self, query)
      File "/home/wking/src/gentoolkit/pym/gentoolkit/query.py", line 51, in __init__
        if not set(query).intersection(needed_chars):
    TypeError: 'Package' object is not iterable

Also use ffmpeg-9999 (for better future-proofing) and update the
length of the resulting deptree.