Support converting sets.Set() objects

[mgedmin]

Fixes #23.

This PR could use some work:

• tests!
• it doesn't work when run on Python 3
• it produces pickles refering to __builtin__.set instead of builtins.set

[mgedmin]

NB: I noticed that the default renamer of '__builtin__ set': 'builtins set' causes calls to ZODB.broken.rebuild() to be embedded in the output pickles, if you run the conversion on Python 2. I don't think that's desired. I've added a commented-out test, but I'm not sure how to fix it. (Stuff a fake builtins module into sys.modules?)

[mgedmin]

I've found a way (an ugly one) of pickling sets so they unpickle correctly on Python 3 without relying on ZODB.broken.

[mgedmin]

I think this PR is ready for review.

[icemac]

LGTM from reading through the code.

[mgedmin]

@icemac: I would like to request a PyPI release with this fix.

I'm happy to help with making the release if you add me (PyPI username mgedmin) as a PyPI maintainer for zodbupdate.

[icemac]

@mgedmin I added you as owner of this package on PyPI.

[mgedmin]

Thank you!