sapphire-py: Add networks RPC URLs to constant

[matevz]

SUMMARY

The map of network name -> RPC URL should be part of the Sapphire Py wrapper.

This is a typical snippet in the current python projects:

    def __init__(self, network_name: str):
        networks = {
            "sapphire": "https://sapphire.oasis.io",
            "sapphire-testnet": "https://testnet.sapphire.oasis.io",
            "sapphire-localnet": "http://localhost:8545",
        }
        self.w3 = setup_web3_middleware(networks.get(network_name), PRIVATE_KEY)

sapphire.NETWORKS constant should be used instead of networks.