Mark Python 2.5 and older code with pragma: nocover

This is maybe cheating a bit, but the effort required for building
ancient Python versions from source just so I could run tests on Travis
is too great (and mocking (un)bound methods with im_self/im_func
attributes on newer Pythons seems somewhat pointless).
Improve test coverage a little bit

I was wrong when I thought only Python 2.5 compat code remained
untested.  There's also that early stop when reaching a module in
show_backrefs(), and all the subprocess spawning logic in show_graph().