workaround a changed path in weasyprint submodule

[dirks]

Fixes #51

Alternative would be to pin the versions of weasyprint

setup.py

    install_requires=[
        'mkdocs>=0.17',
        'weasyprint>=0.44,<0.53',
        'beautifulsoup4>=4.6.3'
    ],

and maybe mention it in
README.md

3. WeasyPrint (Version >=0.44 to <0.53) depends on cairo, Pango and GDK-PixBuf which need to be installed separately. Please follow your platform installation instructions carefully:
    - [Linux][weasyprint-linux]
    - [MacOS][weasyprint-macos]
    - [Windows][weasyprint-windows]

[claudep]

+1, this is needed... and sufficient to produce a PDF with weasyprint >= 0.53.

[gsalvatella]

@comwes could we merge this?