Misc py2/py3 related upgrades
Misc py2/py3 related upgrades

Includes following commits from Bitcoin Core:
777799 - [qa] Fix pyton syntax in rpc tests
fab389 - [qa] rpc-test: Normalize assert()
fa524d - [qa] Use python2/3 syntax
fa2cea - [qa] rpc-tests: Properly use integers, floats
faa41e - [qa] py2: Unfiddle strings into bytes explicitly

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