Skip processing svg anchor elements

[saulshanabrook]

EDIT: Also fixes #5610

Fixes #5589 by skipping
processing of a elements that are not HTMLAnchorElements, including
those within SVG elements. Currently, if you have a a element
inside an SVG element it will break HTML rendering since that element
does not have a href like other a elements do.

Screenshots

Before, this code (from issue #5589) will end up raising an exception while rendering and so will display no output:

from IPython.display import display, SVG

class A:
    def __init__(self):
        print("init A")
        
    def _repr_html_(self):
        return """
<svg height="44pt" viewBox="0.00 0.00 62.00 44.00" width="62pt">
<g class="graph" id="graph0" transform="scale(1 1) rotate(0) translate(4 40)">
<g class="node" id="node1">
<title>a</title>
<g id="a_node1"><a xlink:title="(0) a">
<ellipse cx="27" cy="-18" fill="#444444" rx="27" ry="18" stroke="#000000"/>
<text fill="#ffffff" font-family="Times,serif" font-size="14.00" text-anchor="middle" x="27" y="-14.3">A</text>
</a>
</g>
</g>
</g>
</svg>
"""
    
a=A()
a

After, this exception will not be raised since we skip processing the a elements in the svg element and so it will render correctly, as it does in Jupyter Notebook

[blink1073]

Thanks!