Fix test failures on Python 3.8.0a1

[mgedmin]

Prompted by zopefoundation/zopetoolkit#32.

[tseaver]

Please add 3.8 to tox.ini and setup.py as well?

[mgedmin]

I do not like to claim Python 3.8 compatibility at this point, because Python 3.8 does not exist yet, and we cannot know 3.8 final won't uncover some other issues.

[icemac]

LGTM and I understand your concerns about not yet claiming Python 3.8 compatibility until a final release exists.

[mgedmin]

@tseaver do you accept my reasons, or should I bite the bullet and make the changes you requested? ;)

[tseaver]

@mgedmin I guess I'm fine with leaving out the Trove classifier, but we should probably have some housekeeping / future issue to add it everywhere once there is a release (maybe a beta, since those should be feature-stable?)