Hi,

This PR is a first big step towards resolving #1326096. I went through the pain to recompile libiconv, libxml2 and libxslt with Visual Studio 2015/ucrt to have binaries that can be used to build a Python 3.5 wheel.

This PR makes sure that the ucrt binaries are downloaded when we are on Python 3.5. I documented the actual compilation of the binaries in a reproducible manner at https://github.com/mhils/libxml2-win-binaries. After merging this PR and installing the Visual C++ Build Tools (or Visual Studio), a Python 3.5 x86 Windows wheel can be build as follows:

> "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat"
> python3 setup.py bdist_wheel --static-deps

I sucessfully tested the resulting wheel on a clean Win7 VM which worked fine.

If I could ask for a favor, it would be great if you could upload a Python 3.5 Windows wheel to PyPi as soon as possible (feel free to take the wheel linked above or compile your own). We're currently migrating @mitmproxy to Python 3 and lxml is currently the only dependency that holds a pip3 install mitmproxy back.

Thanks!
Max