wendelin.core: Add -Python3 tests

X tests fail as e.g.

    >>> test.py/fs-!wcfs
    $ make test.py # WENDELIN_CORE_TEST_DB=<fs>
    python setup.py ll_build_ext --inplace
    running ll_build_ext
    copying build/lib.linux-x86_64-3.7/wendelin/bigfile/_bigfile.cpython-37m-x86_64-linux-gnu.so -> bigfile
    python -m pytest --ignore=3rdparty --ignore=build --ignore=t
    ============================= test session starts ==============================
    platform linux -- Python 3.7.7, pytest-4.6.11, py-1.9.0, pluggy-0.13.1
    rootdir: /srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/wendelin.core-dev
    make: *** [Makefile:168: test.py] Aborted
    error   test.py/fs-!wcfs        1.639s  # 0t 1e 0f 0s

They pass under tox.

component/wendelin.core/test-py3.cfg

+# SlapOS software release to test wendelin.core/py3 on Nexedi testing infrastructure.
+
+[buildout]
+extends = test.cfg
+
+[python]
+part = python3
+
+
+[versions]
+# default zope.testing 3.9.7 raises SyntaxError on py3
+# TODO v↑ default in stack/slapos.cfg
+zope.testing = 4.7
+
+# XXX the same numpy version as defined in component/numpy/
+# FIXME software/jupyter pins numpy to 1.13.1 which is incompatible with python3.
+#       However there numpy cannot be upgraded easily - see commit fce56242 by @tomo.
+# TODO -> fix default numpy to work everywhere
+numpy = 1.16.4

component/wendelin.core/test-zodb5-py3.cfg

+# SlapOS software release to test wendelin.core/ZODB5/py3 on Nexedi testing infrastructure.
+
+[buildout]
+extends = test-zodb5.cfg
+
+[python]
+part = python3