Make annotations less fragile

[mdickinson]

The annotations code is somewhat fragile: it makes assumptions that are often violated by real-world code (for example, that every bound method in Python 2 will have a non-None im_class attribute, or that every function will have a __name__ attribute).

It would probably be worth doing an except Exception around each annotation, logging any exceptions, and continuing.

[VojtechFried]

One more thing to consider: expired weakrefs. E.g.
File "C:\Python36\lib\site-packages\refcycle\object_graph.py", line 339, in export_json
self.annotated().export_json(filename=filename)
File "C:\Python36\lib\site-packages\refcycle\object_graph.py", line 279, in annotated
for vertex in self.vertices
File "C:\Python36\lib\site-packages\refcycle\object_graph.py", line 279, in
for vertex in self.vertices
File "C:\Python36\lib\site-packages\refcycle\annotations.py", line 172, in object_annotation
if isinstance(obj, BASE_TYPES):
ReferenceError: weakly-referenced object no longer exists

[mdickinson]

Nice catch; thanks. Do you happen to have a short snippet that reproduces (and that I can turn into a testcase)?

[mdickinson]

Opened #78 for the bug reported by @VojtechFried.