This commit introduces initial support for Etherscan API V2.

Key changes include:

- Updated the core `Client` in `etherscan/client.py`:
    - Base API URL changed to `https://api.etherscan.io/v2/api?`.
    - Added handling for the `chainid` parameter, making the client
      aware of different Ethereum chains. `Client.__init__` now accepts
      a `chain_id` (defaults to 1 for Mainnet).
- Adapted `etherscan/client.ropsten.py`:
    - The Ropsten client now internally uses the main V2 `Client`
      configured with `chain_id=3`.
    - A `DeprecationWarning` is issued to encourage you to migrate
      to the main client with the appropriate `chain_id`.
- Updated module-specific classes (`Account`, `Blocks`, `Contract`):
    - Their `__init__` methods now accept an optional `chain_id`
      parameter (defaulting to 1) and pass it to the base `Client`.
      This allows instantiation of these classes for specific chains.

Work on updating other module-specific classes (gas_tracker, logs, proxies, stats, tokens, transactions) is in progress. Further steps will involve completing these updates, reviewing all API endpoints against V2 documentation, and updating examples and tests.