Drop Python 3.6 support?

[mauritsvanrees]

The plone52-py36 job currently fails, probably because I released Plone 5.2.11 yesterday. Sample failure:

Version and requirements information containing charset-normalizer:
  [versions] constraint on charset-normalizer: 2.1.1
  Requirement of requests: charset-normalizer~=2.0.0
While:
  Installing test.
Error: The requirement ('charset-normalizer~=2.0.0') is not allowed by your [versions] constraint (2.1.1)

This is the kind of version conflict that I am not solving anymore in Plone coredev 5.2 for Python 3.6: this Python version is out of support, and Plone dropped support already in 5.2.10.

I propose that we drop support for python 3.6 in collective.exportimport. @pbauer Would that be okay with you?

Workaround might be to add something like this, but it is untested and could give a different conflict:

[versions:python36]
charset-normalizer = 2.0.0

[t7abhay]

Looks good to me :)