Latest version 0.9.7.1 break Marshmallow apispec dumping

[overbost]

Maybe it's just an incompatible versions, but i must use this set of packages

apispec==5.2.2
marshmallow==3.15
flasgger==0.9.5

i can't upgrade apispec or marshmallow because there are other dependency.

with flasgger 0.9.7.1

TypeError: Object of type SchemaMeta is not JSON serializable

I suggest to:

• fix version requirements with apispec/marshmallow
• write versions changelog
• better error output when generating apispec. It's struggle to find which API cause the problem. I use swag_from decorator

Thanks