Drop support for ZODB3

Nexedi stack is dropping support for that old ZODB version - see e.g.

- nexedi/slapos@70d05199
- nexedi/neoppod@3a8f6f03
- nexedi/wendelin.core@0802da2b

Regarding test/gen_testdata.py: even though ZODB4 uses zodbpickle, and
so should be able to load pickles encoded with protocol 3 even on
python2, in practice it does not work so well: ZODB4 tests fail if I set

    --- a/src/ZODB/_compat.py
    +++ b/src/ZODB/_compat.py
    @@ -34,7 +34,7 @@
         HIGHEST_PROTOCOL = cPickle.HIGHEST_PROTOCOL
         IMPORT_MAPPING = {}
         NAME_MAPPING = {}
    -    _protocol = 1
    +    _protocol = 3
         FILESTORAGE_MAGIC = b"FS21"
     else:
         # Python 3.x: can't use stdlib's pickle because

-> so continue to preserve protocol < 3 when generating the test
database for compatibility - now with ZODB4/py2.

/reviewed-by @jerome
/reviewed-on nexedi/zodbtools!18