Update py-cairosvg to 1.0.20.

Version 1.0.20, released on 2016-02-23
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* Allow the user to give parent size

Version 1.0.19, released on 2015-10-30
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* Drastically improve the performance of ``Node()``

Version 1.0.18, released on 2015-10-20
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* Use cairo groups to apply filters

Version 1.0.17, released on 2015-10-09
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* Fix scale and position of markers

Version 1.0.16, released on 2015-08-05
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* Support the text-rendering property

(kleink)