Chain enum and UnsupportedChainException (python)

[antazoey]

Summary

Creates chain constants such as "mainnet" or "goerli" now accessible as Chain.MAINNET or Chain.GOERLI. Replaces the use of these constants through the code to use the available constants.

Other Information

This afford Pycharm type hints and allows the same constants to not have to be redefined down stream in apps/libs that use this library. It also makes the code look more polished.

Finally, if your pull request affects documentation or any non-code
changes, please explain.

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I updated the documentation to show using these constants in cases where the chain name was being specified.

[antazoey]

Questions about the documentation, end of line 71 in in3.model.py, about the config parameter chain_finality_threshold:

chain_finality_threshold (int):  Behavior depends on the chain consensus algorithm: POA - percent of signers needed in order reach finality (% of the validators) i.e.: 60 %. POW - mined blocks on top of the requested, i.e. 8 blocks. Defaults are defined in enum.Chain.

1.). Where is this enum.Chain? I can't find t.
2.) Why are consensus algorithms defined in an enum with name Chain? The rest of the library refers to Chain as being a string such as mainnet or goerli. Is this a typo? If so, I will fix it. And then what is the name of the actual enum for Consensus Algorithms?
3.) Why are there chains in the chain_configs that are not yet used anywhere? I left them out in this PR from the enum, thinking that they can be added once IPFS support is added, etc.

I actually suspect this is referring to chain_configs. I will look at more documentation to find the use of those configs...