lxml not by default on windows

[3nids]

Some windows users are reporting that lxml is missing. Any chance to use the system ElementTree: https://docs.python.org/3/library/xml.etree.elementtree.html

They seem to be compatible: https://lxml.de/1.3/compatibility.html

ping @danjov this was added in #13

[3nids]

[danjov]

@3nids Thanks for reporting. We'll take a look into it and replace lxml with ElementTree.

[3nids]

Thanks!