Summary

This fixes the loose type signature that relied on typing.Any in Headers.get with a type-safe signature that directly matches the parent MutableMapping[str, str] definition and leads to a concrete return type.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
  • Ref: `Headers.get` typing signature is annoyingly loose #1120
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
  • The tests don't seem to apply to this change, as it's just a typing signature change
• I've updated the documentation accordingly.
  • Does not seem to apply for this change