Removed support for the 'bytes_as_strings' arguments to pickling APIs.

- Pickles created when that argument was true might not be unpickled
  without passing 'encoding="bytes"', which ZODB couldn't reliably enforce.

- On Py3k, ZODB will be using 'protocol=3' pickles anyway.
Merge branch 'master' into remove-bytes_as_strings
Unused import.
Merge branch 'master' into remove-bytes_as_strings
Merge master.