Allow downstream type checking

[daneah]

This change is a: (check at least one)

• Bugfix
• Feature addition
• Code style update
• Refactor
• Release activity

Is this a breaking change? (check one)

• Yes
• No

Is the:

• Title of this pull request clear, concise, and indicative of the issue number it addresses, if any?
• Test suite passing?
• Code coverage maximal?
• Changelog up to date?

What does this change address?

This package has type hints, which are checked for self-consistency. Downstream packages may have type hints, also checked for self-consistency. Without a py.typed file, downstream applications can't check for consistency against this package.

How does this change work?

Create a py.typed file and include it via MANIFEST.in.

Additional context

Updated sphinx because the docs were failing to build against Python 3.10.