Fail to import if both pydantic or msgspec are unavailable

[lordmauve]

Version: quart-schema==0.20.0

For unknown reasons, the pydantic dependency in my project was specified directly in project.requirements rather than as quart-schema[pydantic]. After finding no usages of pydantic in the project this dependency was cleaned up.

This led to the baffling situation where various views were just serving HTTP 400, with a generic

Bad Request

The browser (or proxy) sent a request that this server could not understand.

I would suggest making the package unimportable if the required dependencies are not satisfied, rather than failing only at the point of performing a validation.