Add setter for self._web3 in SlitherReadStorage

[webthethird]

Adds setter for self._web3 in SlitherReadStorage for more flexibility using SRS in other tools.

SlitherReadStorage._web3 depends on SlitherReadStorage.rpc, which is not protected, so it would seem simple enough to just set the RPC if we want to set up our own Web3 provider.

However, in my project I already have a Web3 provider created, and I need to do the following to handle PoA networks:

        # Workaround for PoA networks
        if is_poa:
            self._w3.middleware_onion.inject(geth_poa_middleware, layer=0)

I would then like to set srs.web3 = self._w3 so that subsequent calls to srs.get_slot_values will work with PoA networks.

I suppose the alternative is for my program to store is_poa for later, and then do the following, though this is only necessary because there is no srs.web3 setter:

        srs.rpc = self._rpc_provider
        if self._is_poa:
            srs.web3.middleware_onion.inject(geth_poa_middleware, layer=0)

[montyly]

Can we add -> None ?

[webthethird]

Done

[webthethird]

I believe we are going a different direction with #1843