Events cannot retrieve their ABI if multiple events have the same name

* Version: 5.12.0
* Python: 3.7
* OS: osx

### What was wrong?

`_get_event_abi()` doesn't work with overloaded events (same name, but different argument counts).

From [here](https://github.com/ethereum/web3.py/blob/aa30b3bf97d67b8434cb399c659d8b6425a9bec1/web3/contract.py#L1125), we can see that `_get_event_abi()` calls `find_matching_event_abi()`, which has the problem of not working if two events of the same name are found.

In particular, consider the following set of overloaded events (found in the Compound `Comptroller` contract for example):

```
/// @notice Emitted when an action is paused globally
event ActionPaused(string action, bool pauseState);

/// @notice Emitted when an action is paused on a market
event ActionPaused(CToken cToken, string action, bool pauseState);
```

Equally `.abi` is not available for factory generated events.

### How can it be fixed?

Expand `find_matching_event_abi` to find the appropriate event through its signature, not through the event name.