Properly test PyPy

[chrysn]

As of PyPy3.5 v5.9.0, aiocoap can basically run on pypy thanks to fixes by Dodan Mihai (recvmsg support and socket error reporting).

Some tests still fail (which needs to be tested on a proper pypy build that actually fits my system), and I did not test the various extensions -- that should be done, and then PyPy support should be documented / announced.

[chrysn]

Just did another test with PyPy3.5 6.0. It passed the test suite with no additional packages installed, and could run the full ./setup.py test (which pulls in all dependencies) without error as well.

Next step is finding a docker image where this can be tested in CI. (I'd prefer not to do a full pypy build within the CI on every single commit.)