Fix zeth helper

[AntoineRondelet]

• Fixed missing conversion to checksum for flag value in zeth-helper eth-fund
• The eth-fund command fails with the following error:

  File "<path-to-zeth>/zeth/client/env/lib/python3.7/site-packages/web3/middleware/validation.py", line 77, in check_extradata_length
    len(result), MAX_EXTRADATA_LENGTH, result
web3.exceptions.ExtraDataLengthError: The field extraData is 495 bytes, but should be 32. It is quite likely that you are connected to a POA chain. Refer to http://web3py.readthedocs.io/en/stable/middleware.html#geth-style-proof-of-authority for more details. The full extraData is: HexBytes('0xf901ecf896d89411345428e41

[AntoineRondelet]

* The `eth-fund` command fails with the following error:

  File "<path-to-zeth>/zeth/client/env/lib/python3.7/site-packages/web3/middleware/validation.py", line 77, in check_extradata_length
    len(result), MAX_EXTRADATA_LENGTH, result
web3.exceptions.ExtraDataLengthError: The field extraData is 495 bytes, but should be 32. It is quite likely that you are connected to a POA chain. Refer to http://web3py.readthedocs.io/en/stable/middleware.html#geth-style-proof-of-authority for more details. The full extraData is: HexBytes('0xf901ecf896d89411345428e41

@dtebbs AFAIK, the implementation of zeth-helper eth-fund must be extended. If I use zeth-helper eth-fund --source-addr <my-funded-address> on Barkerloo, then we don't enter this block https://github.com/clearmatics/zeth/blob/develop/client/zeth/helper/eth_fund.py#L39 and we don't set the Geth PoA middleware which triggers this error. So we probably want to extent this file to add a case for the bakerloo network name (which is the default name given by running zeth-helper eth-gen-network-config bakerloo).
Surely, if people decide to point to bakerloo with a different network name (or point to, say, Eth mainnet and name the network "bakerloo" for some reason), the right middleware won't be set anymore and the error will show up again, but at least, someone copying and pasting commands int he Bakerloo documentation would not have this error anymore. We can also add further documentation here to document this edge case.

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[EDIT] Instead of bothering extending this command, which is only relevant when used in a test environment like Ganache or "autonity-helloworld", I simply extended the documentation of the command to clarify the intent and contexts in which it is relevant. Also, I switched to middleware_onion (see ethereum/web3.py#1210) since middleware_stack was still used and thus broke after the upgrade to web3 >5 in #389 (see: https://web3py.readthedocs.io/en/stable/releases.html#v5-0-0-alpha-4)

[dtebbs]

LGTM