devel/py-swagger-spec-validator: Update to 2.4.3

While I'm here:

• Pet portlint

Changes since 2.3.1:

• Disallow multiple types in schema definitions
• Add warning when using "$ref" together with other properties
• Ensure that only valid references ("$ref" attribute is present with string value) are dereferenced
• Add warning when $ref is defined to have "None" value
• Fix regression, introduced in 2.4.2, that was causing descending references with no scope
• The changes can be also retrieved at:

https://github.com/Yelp/swagger_spec_validator/blob/v2.4.3/CHANGELOG.rst

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D19312