removal of dependency datamodel-code-generator

[ricardogsilva]

Pygeoapi currently has a dev dependency on datamodel-code-generator. This third-party dependency is never used by pygeoapi at runtime. Moreover, it was only used when autogenerating the models in pygeoapi/models/cql.py.
Since its creation, this file that contains the autogenerated models has even had some manual modifications, meaning it has diverged from the automated generation process - thus making it harder to ever regenerate it, if needed.

Keeping the dependency on datamodel-code-generator also has the downside that it actually prevents pygeoapi from moving to a newer version of the jsonschema package[1] - due to this dependency, pygeoapi's jsonschema is stuck at v4.17.3, and this is the last version to support Python 3.7, which pygeoapi still aims to support (however, check #1360 for more info).

As such, I am proposing that datamodel-code-generator be removed from pygeoapi's requirements-dev.txt file.

[1] - datamodel-code-generator depends on openapi-schema-validator and on openapi-spec-validator, both of which are not used by pygeoapi, and both of which mandate jsonschema < 4.18.0. The current version of jsonschema` is v4.19.1

[ricardogsilva]

Completed by merging #1370