Manually sort XML attributes

[vojtechtrefny]

Since Python 3.8 attributes are no longer automatically sorted,
see https://bugs.python.org/issue34160

---

This was originally reported in #51 but I wasn't able to find the fix.

The test_pretty_xml test case is still failing on 3.8 even with this. Looks like running parseString and toprettyxml now moves the XML Namespace attributes. I don't know if this is intentional or if it can be considered bug in Python. I'd personally probably just move the xmlns attributes in the test to match the expected result but I'm not an XML/SVG expert.

Failed test with Python 3.8:

======================================================================
FAIL: test_pretty_print (test_pretty_xml.TestPrettyXML)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builddir/build/BUILD/svgwrite-1.3.0/tests/test_pretty_xml.py", line 20, in test_pretty_print
    self.assertEqual(e, r)
AssertionError: '<svg baseProfile="full" height="100%" version=[164 chars]nk">' != '<svg xmlns="http://www.w3.org/2000/svg" xmlns:[164 chars]0%">'
- <svg baseProfile="full" height="100%" version="1.1" viewBox="0,0,10000,10000" width="100%" xmlns="http://www.w3.org/2000/svg" xmlns:ev="http://www.w3.org/2001/xml-events" xmlns:xlink="http://www.w3.org/1999/xlink">
+ <svg xmlns="http://www.w3.org/2000/svg" xmlns:ev="http://www.w3.org/2001/xml-events" xmlns:xlink="http://www.w3.org/1999/xlink" baseProfile="full" height="100%" version="1.1" viewBox="0,0,10000,10000" width="100%">

[mitar]

Shouldn't sorting be now done only if pretty-print is enabled?

[vojtechtrefny]

Sorting is now (Python 3.8) disabled everywhere, including toprettyprint. My idea here was to simply keep current behaviour.

[mitar]

Yea. But current behavior is in my view a workaround around the fact that the order before Python 3.6/3.5 was not stable. So sorting makes it stable. Now, the output is stable without sorting. So sorting could be seen as pretty-printing feature.