Update deploy example in docs

[wolovim]

What was wrong?

A docs example contained a stale (deprecated) deploy() reference. I ran through the example locally and found I was unable to get off the ground without some other cleanup too.

Closes #1679

How was it fixed?

• replaced deploy() with constructor().transact()
• added PyEVMBackend. Without it, you get ValueError: RPC Endpoint has not been implemented: eth_estimateGas.
• latest solidity requires emit keyword to trigger events
• added py-solc-x for latest solidity compilation
• other cleanup: formatting, etc. (some done by black)

Todo:

• Add entry to the release notes

Cute Animal Picture

[pipermerriam]

This would be something I would expect to be caught by doctest. I would recommend doing some investigating to 1) enable doctest on these examples if it isn't enabled or 2) figure out why doctest isn't catching these if it is already enabled.

[wolovim]

Will check into bolstering these examples via doctests next. Shipping this update to get the next beta release out