DRY up common code in contract classes

[fselmo]

Working on #3540, I noticed a lot of the code that was cleaned up there can ideally be removed from the contract.py and async_contract.py into base_contract.py since the code is nearly identical. The biggest issue I can see to resolve may be the typing, which is why this warrants a separate issue since it's out of the scope of that PR.

Investigate the following, and perhaps other cases:

• refactor __call__() in ContractFunction (+ respective async code) to BaseContractFunction
• refactor __getattr__(), __getitem__() and __iter__() in ContractEvents and ContractFunctions (+ respective async code) to BaseContractEvents and BaseContractFunctions, respectively.