Add a py.typed file, to make types available downstream

[brianhelba]

Description

This will not change any behavior of this package, but it is required for consuming projects to read any existing type information here.

This file is required to exist and should be empty. There's no alternative mechanism. See: https://blog.whtsky.me/tech/2021/dont-forget-py.typed-for-your-typed-python-package/

This follows #2227.

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[tim-schilling]

If we don't have everything using type annotations, is that going to cause problems for someone using mypy?

[brianhelba]

If we don't have everything using type annotations, is that going to cause problems for someone using mypy?

No, not at all, it'll be the same as someone with local code / functions that aren't fully typed. In short, MyPy will infer untyped things as typing.Any. Indeed, this is already how MyPy downstream is perceiving everything in django-debug-toolbar, since we lack py.typed. The addition of this file will be strictly additive, allowing types to be used from this package where they do exist.

Personally, I think it's a nuisance that PEP 561 makes us include this, but it's just become a routine practice.

Also, note that hatchling.build will include py.typed in sdists without any additional configuration.

[tim-schilling]

Thank you for that explanation @brianhelba! I'll merge this once the docs/linter passes.

[github-actions]

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