Transition to starlette 1.0

This PR is part of the transition to starlette 1.0, which introduced breaking changes. Some reverse dependencies are currently failing autopkgtests due to this transition.

This PR depends on other changes in the stack. Recommended merge order:

1. python-pathable - python-pathable!1 (merged)
2. python-jsonschema-path - python-jsonschema-path!1 (merged)
3. python-openapi-schema-validator (this PR)
4. python3-openapi-spec-validator
5. fastapi (Waiting ITP - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1132513)
6. python-openapi-core

On debusine, autopkgtests, for reverse dependencies, for this package may fail until dependencies above are updated. https://debusine.debian.net/debian/developers/work-request/555470/