Use sphinx-svg2pdfconverter as SVG handler instead.

[cbalint13]

Again, fix the readthedocs PDF generation latest failure. All pass on local make latexpdf with inkscape installed.

• Use sphinxcontrib-svg2pdfconverter that picks up inkscape/rsvg-convert instead of imagemagick
• With inkscape such conversion errors like must specify image size will go away on blank SVG drawings.

I hope it is last, also hope that readthedocs's docker image are smart to have inkscape along with imagemagick deps.

Cc @proppy , lets see now.

[proppy]

@cbalint13 thanks for figuring that out, that looks like an even better fix than #84.