Consider pinning drf-yasg in setup.py `install_requires`

[kdmccormick]

Different versions of drf-yasg may break or change the behavior of api-doc-tools.

Thus, if drf-yasg is upgraded in a service, it may seem like api-doc-tools changed, even if that package's version did not change.

I recommend that we require a specific version of drf-yasg as an installation requirement of edx-api-doc-tools. Ideally, this pin could be continuously updated by make upgrade PRs in this repo.

As far as I know, no edX repository other than this one directly depends on drf-yasg, so we shouldn't see issues with conflicting version constraints.