Drop Python 3.4 support. Add 3.7 and 3.8 support. [3.x]

[mauritsvanrees]

Same as current master. No code changes. Problem with 3.4 is that we get an lxml version which requires Python 2.7, 3.5 or later

Note that Plone will keep using 3.x for the foreseeable future, because we want the subobject support.