Fix a bug occurring when setting python3.6 as ${buildout:executable} in profiles.

************ REBOOTSTRAP: IMPORTANT NOTICE ************
bin/buildout is being reinstalled right now, as new python:
  /srv/slapgrid/slappart14/srv/testnode/cup/soft/4281ee9e21ffccb8d017eba71a9cd9b1/parts/python3.6/bin/python3.6
is available, and buildout is using another python:
  /opt/slapgrid/4cca7f1c7db00b5e7a4348f8df5c24ab/parts/python2.7/bin/python2.7
Buildout will be restarted automatically to have this change applied.
************ REBOOTSTRAP: IMPORTANT NOTICE ************

networkcache: Downloading pypi-index-96af89cdde216340df5aec1e9a1392e0-setuptools==40.4.3...
networkcache: ignored unhandled exception at /srv/slapgrid/slappart14/srv/testnode/cup/soft/4281ee9e21ffccb8d017eba71a9cd9b1/eggs/slapos.libnetworkcache-0.20-py2.7.egg/slapos/networkcachehelper.py:104
HTTPError: HTTP Error 404: Not Found
networkcache: Downloading pypi-index-99e2b3234a294248d07c2bb8e6698302-setuptools==40.4.3...
networkcache: ignored unhandled exception at /srv/slapgrid/slappart14/srv/testnode/cup/soft/4281ee9e21ffccb8d017eba71a9cd9b1/eggs/slapos.libnetworkcache-0.20-py2.7.egg/slapos/networkcachehelper.py:104
HTTPError: HTTP Error 404: Not Found
networkcache: Downloading pypi-index-a46be9bd3756ada5c303353adbb6407f-setuptools==40.4.3...
networkcache: ignored unhandled exception at /srv/slapgrid/slappart14/srv/testnode/cup/soft/4281ee9e21ffccb8d017eba71a9cd9b1/eggs/slapos.libnetworkcache-0.20-py2.7.egg/slapos/networkcachehelper.py:104
HTTPError: HTTP Error 404: Not Found
networkcache: Downloading pypi-index-45c869bb967e16b5e30ed107541a36be-setuptools==40.4.3...
networkcache: ignored unhandled exception at /srv/slapgrid/slappart14/srv/testnode/cup/soft/4281ee9e21ffccb8d017eba71a9cd9b1/eggs/slapos.libnetworkcache-0.20-py2.7.egg/slapos/networkcachehelper.py:104
HTTPError: HTTP Error 404: Not Found
networkcache: Downloading slapos-buildout-c4345cb200e7d70be4984e98a0864d72...
Downloaded https://files.pythonhosted.org/packages/6e/9c/6a003320b00ef237f94aa74e4ad66c57a7618f6c79d67527136e2544b728/setuptools-40.4.3.zip#sha256=acbc5740dd63f243f46c2b4b8e2c7fd92259c2ddb55a4115b16418a2ed371b15 from network cache.
networkcache: Downloading pypi-index-0b71f171beaaa2eb14aa9fd650059e50-zc.buildout==2.7.1+slapos001...
Downloaded https://pypi.org/simple/zc.buildout/ from network cache.
networkcache: Downloading slapos-buildout-fd4ac0343c559873f75e8e4be0b06366...
Downloaded http://www.nexedi.org/static/packages/source/slapos.buildout/zc.buildout-2.7.1+slapos001.tar.gz from network cache.
networkcache: Downloading pypi-index-f0a660f10b8bd0b2f65992c829d98404-slapos.libnetworkcache==0.20...
Downloaded https://pypi.org/simple/slapos.libnetworkcache/ from network cache.
networkcache: Downloading slapos-buildout-b7f445bbec09ae840c83ded70a305e7c...
Downloaded https://files.pythonhosted.org/packages/b4/75/ae13749622d6ace9dae2442399d955d29241cfded7b48245320e2c0953b1/slapos.libnetworkcache-0.20.tar.gz#sha256=a6ceb542499fb572641abb1c7b675329bb235fda43d2f28b541906179e8db8db from network cache.
Traceback (most recent call last):
  File "<string>", line 67, in <module>
  File "<string>", line 64, in main
  File "<string>", line 30, in setup_script
  File "/srv/slapgrid/slappart14/srv/testnode/cup/soft/4281ee9e21ffccb8d017eba71a9cd9b1/eggs/zc.buildout-2.7.1+slapos001-py3.6.egg/zc/buildout/easy_install.py", line 1313, in scripts
    sname = os.path.join(dest, sname)
  File "/srv/slapgrid/slappart14/srv/testnode/cup/soft/4281ee9e21ffccb8d017eba71a9cd9b1/parts/python3.6/lib/python3.6/posixpath.py", line 80, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType
While:
  Installing.

This also fixes test suite to support python3. When running on python3.6 the problem occurred, but not with python3.5.