[v6] Update provider explicit exports for `__all__`

[fselmo]

What was wrong?

closes #3396 together with #3409

How was it fixed?

• Adds AsyncEthereumTesterProvider to web3/__init__.py.
• Adds EthereumTesterProvider and AsyncEthereumTesterProvider to web3/providers/__init__.py.
• Update __all__ in web3/__init__.py to include all provider classes, including base classes.
• Add __all__ for web3/providers/__init__.py including all explicit exports.

Todo:

• [x] Clean up commit history
• [x] Add entry to the release notes

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