Add lxml to dependencies

[n0aaz]

#41 : as the dependencies for this package are found by setuptools.find_packages(), we need to make lxml an import for it to be a dependency.

[PaleNeutron]

I think, https://github.com/dexplo/dataframe_image/blob/ff1fa4853c480cda1021229e009cdd6e0740c915/setup.py#L18 this line tell setuptools find package (which means dataframe_image here), install_requires is which you need to add lxml.

refer to: https://setuptools.pypa.io/en/latest/userguide/dependency_management.html