Remove the find_global/server_find_global shims

[tseaver]

They exist only for Python2 compatibility (and not at the level of pickle opcodes). Nothing in the Python3 stdlib knows about any find_global, which was historically (in Python2) just a way to expose the C-level find_class. Stuffing a new implementation into the module global hasn't had any effect on how cPickle_pickle_33 actually worked since we dropped Python2 support.

$ rgrep find_global /opt/src/Python-3.3.7/ && echo FOUND || echo NOT FOUND
NOT FOUND

See https://github.com/zopefoundation/zodbpickle/pull/74/files

[tseaver]

See PR #237.