Allow publishing of source on BscScan

[jbargu]

What I did

BscScan (Binance Smart Chain) is an exact copy of Etherscan. The only thing preventing from publishing the source to BscScan is the etherscan substring check in contract.py::publish_source method.

How I did it

Add a check for bscscan substring in the explorer URL check.

How to verify it

Deploy a contract on BSC mainnet with publish_source=True and explorer set to bscscan:

  ├─binance-mainnet
  │ ├─id: binance-mainnet
  │ ├─chainid: 56
  │ ├─explorer: https://api.bscscan.com/api
  │ └─host: https://bsc-dataseed1.binance.org

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases (no existing tests for explorer tests)
• I have updated the documentation
• I have added an entry to the changelog

[Netherdrake]

@iamdefinitelyahuman please consider merging this

[iamdefinitelyahuman]

Looks good, I think this should be in the same release as #961