Avoid CairoSVG-2.0.0rc* on Python 2
Avoid CairoSVG-2.0.0rc* on Python 2

It's funny but:
```
pip install --upgrade 'CairoSVG<2'
```
would pull CairoSVG-2.0.0rc6, while
```
pip install --upgrade 'CairoSVG<2.0.0'
```
would pull the 1.x branch (1.0.22 at time of writing).
Kozea/WeasyPrint,Kozea/WeasyPrint