Add support for testnets

[litneet64]

Hi, I'm working on a project that relies on etherscan-python for communicating with Etherscan.io's API, and having support for querying stuff on the other testnets will come really handy for the end goal of it.

Luckily, I already added that feature on a fork I did from this project following the overall "minimalism" (as said on the description) and also adding the testnets to the module tests with their own working/stable test cases (randomly sampled - [*]) along with a minimalist usage example on the README.

[*] Unfortunately, I don't possess a PRO API key, so I couldn't find working test cases for the PRO endpoints nor I know if they are already passing with those parameters or not.

[github-actions]

Thanks for reporting this issue, we are on it :)

[litneet64]

Also, logs for testnets are not included as I think their existence on this repo is a decision for you to take, however their proper generation is already found on the module tests' code.

[pcko1]

@litneet64 apologies for the late reply, I somehow missed this notification. I have just accepted your PR! Unfortunately the pro methods are not supported for the test-nets by Etherscan.io but everything else seems to be working well. Thanks a lot for this contribution :)