Migrate away from abandoned coreapi/coreschema dependency

[johnthagen]

Upsteam issues:

• https://github.com/axnsan12/drf-yasg/issues/389
• https://github.com/axnsan12/drf-yasg/issues/473

From the core-api repo:

DEPRECATION NOTICE: As of djangorestframework 3.9, the plan is to phase out CoreAPI in favor of OpenAPI as the default schema representation. New projects should consider using OpenAPI rather than CoreAPI. See the DRF 3.9 release announcment for more details.

DRF will soon be raising deprecation warnings for CoreAPI schemas and they will be removed in DRF 3.14. I haven't looked closely enough at drf-yasg2 to know if this will affect this package, or if we only rely on coreapi directly rather than through DRF itself.

The coreapi Python package is frozen and has not seen a release in 3 years. It was last tested on Python 3.6. This is a liability for the long term health of this project. This upstream issue would have broken drf-yasg[2] on Python 3.10, but a maintainer took over one of coreapi's frozen dependencies, itypes and fixed the issue.

Another benefit of removing the dependency on coreapi is that coreapi depends on requests, which depends on chardet. chardet is LGPL licensed which presents problems for developers wanting to package their application as a binary using PyInstaller or PyOxidizer (https://github.com/psf/requests/issues/4417).

Seems like we should try to identify and migrate over the parts of coreapi that are needed for drf-yasg2.

coreapi also brings in coreschema which is used by drf-yasg2. coreschema has also not been maintained for 3 years.

[johnthagen]

coreapi

https://github.com/JoelLefkowitz/drf-yasg/blob/a0f6f9de0514c25ef99ad906c1f2ca3c7d159f86/setup.py#L14

https://github.com/JoelLefkowitz/drf-yasg/blob/a0f6f9de0514c25ef99ad906c1f2ca3c7d159f86/src/drf_yasg2/openapi.py#L7

https://github.com/JoelLefkowitz/drf-yasg/blob/a0f6f9de0514c25ef99ad906c1f2ca3c7d159f86/src/drf_yasg2/codecs.py#L6

https://github.com/JoelLefkowitz/drf-yasg/blob/a0f6f9de0514c25ef99ad906c1f2ca3c7d159f86/src/drf_yasg2/generators.py#L8

coreschema

https://github.com/JoelLefkowitz/drf-yasg/blob/a0f6f9de0514c25ef99ad906c1f2ca3c7d159f86/setup.py#L15

https://github.com/JoelLefkowitz/drf-yasg/blob/a0f6f9de0514c25ef99ad906c1f2ca3c7d159f86/src/drf_yasg2/inspectors/query.py#L3