Fix AttributeError on Python 3.9 due to usage of removed API in xml

[kitchoi]

Closes #490

The first commit fixes errors that can be seen by running the test suite on Python 3.9.

The second commit makes similar changes, but those changes are not covered by tests.

On the changes that are not covered by test:
For enable.svg.document.addRectToDocument. I was trying to write a test that does something like this:

    def test_add_rect_to_document_with_clip_path(self):
       # create_svg_text is a function that adds the necessary XML declaration and <svg> root element
        svg_text = create_svg_text(
            '<rect x="11" y="11" clip-path="circle() view-box" />'
        )
        document = document.SVGDocument(etree.parse(StringIO(svg_text)).getroot(), renderer=KivaRenderer())

However it still does not reach the code path where I fixed the usage of ElementTree.
It seems that if 'clip-path' in node: is always false. Previously it was doing if 'clip-path' in node.keys() where keys() returns the XML attributes, but node was thought to be a dict during a migration from Python 2 to 3 (see #284). Upon fixing in node to in node.keys(), I run into another difficulty trying to open an SVG referenced by the clip-path assuming it was a local file. So I eventually gave up. I am happy to revert the second commit owing to the fact it is untested.

[jwiggins]

Less broken than before... Let's iterate as needed here.

[kitchoi]

Thanks. I will open an issue for the clip-path code path being unreachable.