Require textwrap3 only in <PY36

[ankostis]

Fix #15 by the use of PEP496 environment markers.

[felixonmars]

It needs some more tweaks:

test/test_ansiwrap.py:3: in <module>
    import textwrap3 as textwrap
E   ModuleNotFoundError: No module named 'textwrap3'

ansiwrap/__init__.py:1: in <module>
    from .core import *
ansiwrap/core.py:11: in <module>
    a_textwrap = imp.load_module('a_textwrap', *imp.find_module('textwrap3'))
/usr/lib/python3.8/imp.py:296: in find_module
    raise ImportError(_ERR_MSG.format(name), name=name)
E   ImportError: No module named 'textwrap3'