Fix usage with a relative input SVG file path

[Lucas-C]

Without this fix, one needs to always pass absolute path to the cairosvg command.

There are probably other ways to fix this, maybe by calling os.path.abspath in cairosvg.parser.Tree.__init__

Quick test:

CAIROSVG_TEST_FILES=struct-image-01-t.svg pytest test_non_regression/test_non_regression.py