openapi3 fails to parse empty securityRequirement

[mdellweg]

The comment in https://github.com/Dorthu/openapi3/blob/master/openapi3/paths.py#L304 claims that a securityRequirement must be a dictionary with exactly one key. But https://swagger.io/specification/#securityRequirementObject specifies that "Security Requirement Objects that contain multiple schemes require that all schemes MUST be satisfied for a request to be authorized.". And it provides an Example with an empty securityRequirement ({}). This example would fail to pass in openapi3.