Fix imports in __init__.py files to explicitly re-export symbols

[lukaspiatkowski]

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What does this PR do?

Fixes #842.
By default imports in __init__.py files are treated by type checkers (like mypy or pyright) as private.
With this PR type checkers start to recognize selected symbols imported in __init__.py files as re-exported, so that code like this:

from datadog import statsd

is no longer treated as importing private implementation details of the library.

Description of the Change

In order to mark symbols as intended to be re-exported you can use one of the special import forms. This change does that by adding from Y import X as X (a redundant symbol alias) as described in official Import Conventions written by The Python Typing Team.

Alternate Designs

As an alternative one might define the __all__ module-level attribute in __init__.py file that lists all the symbols intended to be re-exported. The symbols listed in __all__ are represented by str. Making sure that this is list up-to-date might become problematic as only type checkers use that information and not every IDE supports recognizing those strings as symbols, making it hard to maintain them.

Possible Drawbacks

One have to remember to keep those re-export up-to-date to keep type checkers happy.

Verification Process

I've checked by running pyright on

from datadog import statsd

if it gives a warning reportPrivateImportUsage.

Additional Notes

Release Notes

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