hooks: Add hook for lxml

[doudz]

lxml has hiddenimports and is not fully embedded when adding it to hiddenimports

hiddenimports = ['lxml'] is not enough
see pyinstaller/pyinstaller#5306

[bwoodsend]

Perfect thanks. We aim to release these monthly so in a few weeks you'll be able to

pip install -U pyinstaller-hooks-contrib

But until then use

pip install -U https://github.com/pyinstaller/pyinstaller-hooks-contrib/archive/master.zip