Python3 support progress

[ouziel-slama]

No description provided.

[ouziel-slama]

@vbuterin, with this PR all tests that pass with Python 2 also pass with Python 3. Here a document with the state off all tests: https://docs.google.com/spreadsheets/d/1jaw7Gb6W_dCtvs2ujE58bmKNg_Ld16VxvXId29vAP2U

[adamkrellenstein]

@heikoheiko, @vbuterin, will you be okay with this PR when it's been rebased?

[heikoheiko]

Hi Adam,
thanks for the work. But we experienced some performance penalties (in the vm) introduced due the compatibility.

Can you make sure, that heavily looped code (like in the vm) does never call any py2/3 compatibility related isinstance.

Also note, that we probably nowhere use unicode in ethereum. It can safely be assumed, that it's either bytes or long. Except for input from json, which should be converted as soon it was deserialized.

see profile_vm.py in the pypy branch for an easy option to profile the code.