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

The patch that provides raw-extension functionality was merged into ZODB 5.6:

https://github.com/zopefoundation/ZODB/commit/2f8cc67a3ba3

So when testing with ZODB5 >= 5.6 the tests will excercise code path
that uses txn.extension_bytes, and when testing with ZODB4 the tests
will excercise code path that work-arounds lack of txn.extension_bytes.

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

-> Use .[test] to refer to them.
https://stackoverflow.com/a/41398850/9456786

/reviewed-by @jerome
/reviewed-on nexedi/zodbtools!14

----

kirr: factor running `zodb ...` into zodbrun + add test for `zodb -h`.

Added test currently passes on py2, but fails on py3:

	out = <_io.TextIOWrapper encoding='UTF-8'>

	    def usage(out):
	        print("""\
	    Zodb is a tool for managing ZODB databases.

	    Usage:

	        zodb command [arguments]

	    The commands are:
	    """, file=out)

	        cmdv = command_dict.keys()
	>       cmdv.sort()
	E       AttributeError: 'dict_keys' object has no attribute 'sort'

	zodbtools/zodb.py:55: AttributeError

It will be fixed in the next patch.

until https://github.com/zopefoundation/ZODB/pull/183 gets merged, let's
run also the tests for this, since we have support for this extension.

also simplify a bit definition as ZODB is common in all versions

----

kirr:

- cover only last 2 py3 releases: 3.6 and 3.7 for now (3.8 is not yet
  released)
- separate ZODB3 as it supports only python2.

Py3 tests are failing for now and we'll be getting them to pass
incrementally - step by step.

Use tox to test with all kinds of ZODB.
With preceding 3 patches tests pass with all versions of upstream ZODB.

TODO: test coverage for both py2 and py3.