Python API: explicitly reexport classes

[adamjstewart]

When using ee in Python, mypy complains with:

error: Name "ee.Image" is not defined  [name-defined]
error: Name "ee.ImageCollection" is not defined  [name-defined]
error: Module "ee" does not explicitly export attribute "ImageCollection"  [attr-defined]
error: Module "ee" does not explicitly export attribute "Filter"  [attr-defined]
error: Module "ee" does not explicitly export attribute "Filter"  [attr-defined]
error: Name "ee.ImageCollection" is not defined  [name-defined]
error: Name "ee.ImageCollection" is not defined  [name-defined]
error: Module "ee" does not explicitly export attribute "Filter"  [attr-defined]
error: Module "ee" does not explicitly export attribute "Filter"  [attr-defined]
error: Module "ee" does not explicitly export attribute "Filter"  [attr-defined]
error: Module "ee" does not explicitly export attribute "Geometry"  [attr-defined]
error: Module "ee" does not explicitly export attribute "EEException"  [attr-defined]
error: Name "ee.ImageCollection" is not defined  [name-defined]
error: Module "ee" does not explicitly export attribute "Geometry"  [attr-defined]
error: Name "ee.ImageCollection" is not defined  [name-defined]

This is because none of these classes are explicitly exported via __all__. This PR fixes that.

[adamjstewart]

@schwehr anything else needed before this PR can be merged?

[adamjstewart]

Trying to close out old PRs. Is this something that is wanted?

[schwehr]

I think it should go into the interface, but there is a worry about unintentional changes to the interface. Please put a typing.TYPE_CHECKING guard around the change.