Note that we only use requirements.txt for running the test suite inside tox.
I didn't add (yet?) unicodedata2 module to 'install_requires' in setup.py.
Python 3.6 does not need it, as it's built-in unicodedata module is the
same as unicodedata2 backport.
PyPy is also excluded because currently there is no pre-compiled wheel
available for that implementation.
this should speed up the Travis and Appveyor builds, as we don't need to compile
Brotli from source, at least on OSX and Windows. Linux will still use the
.tag.gz source distribution.
