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

On debusine and CI, builds fail due to the missing python-pathable, python-jsonschema-path and python-openapi-schema-validator. Autopkgtests for reverse dependencies of this package may fail until the dependencies mentioned above are updated.

https://debusine.debian.net/debian/developers/work-request/559123/

The build can be reproduced with the newer upstream version of python-pathable, python-jsonschema-path and python-openapi-schema-validator, for example:

gbp buildpackage --extra-package=python3-pathable_0.5.0-1_all.deb --extra-package=python3-jsonschema-path_0.4.5-1_all.deb --extra-package=python3-openapi-schema-validator_0.8.1-1_all.deb