pypy3 support

[heikoheiko]

The major challenge with py3 is to keep the code base clean.
The major challenge with pypy is that some c-module won't work and we'll either have to find a substitute or develop our own bindings.

For reference:
ethereum/pyethereum#132
ethereum/pyethereum#188

note: miniupnpc isn't used anymore.

for pypy3 we need gevent 1.1, currently it breaks neworking:
ethereum/pydevp2p#2

sha3 replacement:
https://pycryptodome.readthedocs.org/en/latest/

own libraries: serpent, ethash

[heikoheiko]

see also #37

[Visgean]

Any progress on this?