Expose typings for the library users

[SergeyTsaplin]

What do these changes do?

These changes expose the type hints for the library users and fix the callbacks interfaces issue. The exposing is described in the PEP561.

Description

If there is no py.typed file in the library root, mypy doesn't discover the type hints from the sources.
So for the following command:

mypy -c "import kopf"
...

fails with message:

<string>:1: error: Skipping analyzing 'kopf': found module but no type hints or library stubs
<string>:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

After the PR will be merged the command above should pass successfully:

mypy -c "import kopf"
Success: no issues found in 1 source file

The change does not affect the end-users and there are no breaking changes.

Type of changes

• Bugfix (non-breaking change which fixes an issue)

Checklist

• The code addresses only the mentioned problem, and this problem only
• I think the code is well written
• Unit tests for the changes that exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt

[lgtm-com]

This pull request introduces 4 alerts when merging d9db887 into 8a08e0a - view on LGTM.com

new alerts:

• 4 for Unused import

[lgtm-com]

This pull request introduces 4 alerts when merging d9db887 into e71dc60 - view on LGTM.com

new alerts:

• 4 for Unused import