RFE: drop using `six` module

[kloczek]

https://adamj.eu/tech/2019/03/13/dropping-python-2-support/

Looking on the code

[tkloczko@devel-g2v scour-0.38.2]$ grep -rw six
scour/scour.py:import six
scour/scour.py:from six.moves import range, urllib
scour/scour.py:    for rid, nodes in six.iteritems(findReferencedElements(doc.documentElement)):
scour/scour.py:        for bucket in six.itervalues(grad_buckets):
scour/scour.py:        sci = six.text_type(length) + 'e' + six.text_type(exponent)
scour/svg_transform.py:from six.moves import range
setup.py:    install_requires=['six>=1.9.0'],
test_scour.py:import six
test_scour.py:from six.moves import map, range
test_scour.py:        desc = six.text_type(doc.getElementsByTagNameNS(SVGNS, 'desc')[0].firstChild.wholeText).strip()
test_scour.py:        desc = six.text_type(doc.getElementsByTagNameNS(SVGNS, 'desc')[1].firstChild.wholeText).strip()
test_scour.py:        desc = six.text_type(doc.getElementsByTagNameNS(SVGNS, 'desc')[2].firstChild.wholeText).strip()
test_scour.py:        desc = six.text_type(doc.getElementsByTagNameNS(SVGNS, 'desc')[3].firstChild.wholeText).strip()
test_scour.py:        desc = six.text_type(doc.getElementsByTagNameNS(SVGNS, 'desc')[0].firstChild.wholeText).strip()
test_scour.py:        class InOutBuffer(six.StringIO, object):
tox.ini:    six

[kloczek]

Only now I found #306.
I've tested it and looks like it works.
Is it possible to commit this PR and release new version? 🤔