Upgrade Web3.py to 5.0.0b4

[saschagoebel]

As discussed we should update Web3.py to a recent version because of the ChainId Bug that has been fixed in version 5.0.0b3.

Sadly there's some compatibility problem with ethpm which I was not able to resolve.

Steps to reproduce

• blockchain-repo branch live-test-fixes
• change web3 to 5.0.0b4 in constraints.txt
• cd tools/bridge
• make test

[ghost]

I've opened a PR: https://github.com/trustlines-protocol/blockchain/pull/263

Make sure you uninstall ethpm first. Details are in the PR.