based on a patch by felix schwarz, thanks!
changes after that:
build msgpack cpp files at release time, do not use cython otherwise.
we want to include .cpp (and .c) files into the sdist, so users
installing the pypi package won't need to have cython installed.
also: python setup.py clean shall also cleanup the *.cpp and binaries
built from that.
.gitignore: ignore generated msgpack .cpp sources
setup.cfg: flake8: ignore 3rd party bundled msgpack source
line length comes from @tw - i would prefer 80 columns personnally, but don't want to argue over that bikeshed
the 120 limit *does* trigger warnings right now, but it's better to do that than to make the problem worse for future PRs
currently adjusted in a way it does not throw errors and warnings with
the source as it is now.
issues will get fixed over time and the exclude list shall get shorter.
