Fix typing in ReadableSpan

[adriangb]

Description

This fixes some incorrect typing in ReadableSpan, including assumptions about the type of attributes which causes a bug when they're not a BoundedAttributes instance.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

This is a refactor so I'm relying on the existing tests

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[adriangb]

There's a lot more of this work that needs to be done across all of the repo(s). Is there any appetite for this from the maintainer team?

[adriangb]

Also I'd skip this from the changelog if possible since it's (mostly) an internal refactor

[srikanthccv]

Only the opentelemetry-api package is in good shape and has CI checks and in the rest of the packages, you see half-complete work. We definitely want to see more parts of the codebase especially public API typed.

[aabmass]

+1 to that, I'd love to have typing working on the SDK. This is tracked in #1608

[aabmass]

One nit on the PR title, seems like we're simplifying some things as well not just updating the type annotations

[adriangb]

@srikanthccv this is ready for review

[adriangb]

Thanks folks!