Include `__all__` in top level `__init__.py`

[johnrezk]

Pylance, the most commonly used Python language server for VS Code, currently displays errors when attempting to access any objects imported in the top level ___init___.py of the edgedb module.

This appears to be due to pylance's distinction between private and public symbols via __all__

In a brief test, I created an __all__ set with public symbols and pylance no longer reported an error.

Considering the ubiquity of VS Code and pylance, it may be a good idea to conform to their standards and include an __all__.

[1st1]

Sure, we can add it. Do you want to submit a PR?