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

[kai]

Summary

While I'm here:

• Pet portlint

Notable 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 are also available at:

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

Test Plan

• poudriere (11.2-, 12.0-RELEASE, 13.0-CURRENT@r344112 amd64 + i386) for each py27 + py36 flavor -> OK
• portlint -> OK
• Runtime tests -> OK

[tcberner]

Looks good to me.