Account for deprecation of DiGraph._repr_svg_ in graphviz (#40)

[scarf005]

replaces Diagraph's _repr_svg() method (deprecated) into _repr_image_svg_xml()

• fixes graphviz's _repr_svg_() deprecation causes error on jupyter notebook #40

[joowani]

Hi @scarf005,

Thank you for reporting this. Could you please do something like below instead to maintain backward compatibility?

try:
    try:
        # noinspection PyProtectedMember
        return str(self.graphviz()._repr_svg_())
    except AttributeError:
        # noinspection PyProtectedMember
    	return self.graphviz()._repr_image_svg_xml()

except (SubprocessError, ExecutableNotFound, FileNotFoundError):
    return self.svg()

[scarf005]

thank you for letting me check for backward compatibility. fixed to code above.

[zhiweio]

@joowani Can you release the PR to PyPI?