Event subscribing, listening, retrieving

[YerimB]

What I did

• Ability to subscribe to any event in a contract by setting a callback to be executed at each new occurrence.
• Ability to listen for any event occurrence with a coroutine.
• Ability to retrieve events from a contract that occurred between two blocks.

Related issue: #1162 #327 #1421

How I did it

general

• Created the ContractEvents class, inheriting from the web3.py class of the same name, and implementing methods to subscribe to, listen for and retrieve events.
• Set a ContractEvents instance ; named 'events' ; to every contract, so it can be used easily.

subscribing

• Created the EventWatcher singleton class with a loop function running in a separate thread, watching for new events.
• Event subscriptions can be added using the corresponding contract instance, using the Contract.events.subscribe method.
• Whenever a new event is detected, the EventWatcher class creates new threads to run the user-defined callbacks instructions.

listening

• Created the Contract.events.listen method, which returns a Coroutine object that can be awaited. A timeout parameter can be passed to the method.
• The Coroutine waits for the user-defined event to occur before returning an web3.datastructures.AttributeDict object containing the event log receipt and a boolean that evaluates to True if the Coroutine has timed out.

retrieving

• Created the Contract.events.get_sequence method, making able to retrieve events from a contract between 2 blocks.
• If no event is specified as parameter, retrieves all contract events between the 2 blocks.

To get events, I am using the web3.py ContractEvent.createFilter and the get_all_entries or the get_new_entries method

How to verify it

Added some tests to the tests/network/test_event.py file.

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation
• I have added an entry to the changelog

[iamdefinitelyahuman]

Wow, impressive work

[YerimB]

Thanks