Add/multipart form data test

[hsundell]

Based on #195

I added also a test for multipart/form-data which seems to fail at the moment.

I encountered this issue when updating a project that had ancient requirements but worked with those.

pyramid-openapi3 <= 0.14.0
openapi-core <= 0.16.7
openapi-schema-validator <= 0.2.3
openapi-spec-validator <= 0.4.0

I don't know exactly what has broken and where, but definitely openapi-core 0.16.8 changed something.

With these versions the form data was processed. Now it fails to deserialize unless I add a custom deserializer which turns the data into json-serializable dict. I wouldn't want to do this for several reasons:

1. Unnecessarily complicated, should work out-of-the-box
2. Requires custom handling of more complicated data types
3. File data in the forms which turn into cgi.FieldStorage objects used to contain streamable BytesIO objects which could be nicely processed in business logic in small chunks. With custom deserializaer I had to convert it into string before OpenAPI validation would pass. Maybe there'd be a better workaround but I'd of course prefer to get the issue fixed instead.

[zupo]

Hey @hsundell, support for the latest openapi-core 0.19 was just merged! Could you please rebase this PR off of latest main so we see what's the latest status with multipart form data? Even if tests now magically pass, I'd still like to add some tests so we catch any regressions in the future.

[am-on]

I think this pull request can be closed since #225 was merged.

• Fix multipart/form-data requests #225 includes test for multipart and bugfix.