Drop no longer used runtime dependency on PyYAML

PyYAML is no longer runtime dependency (7bca8cd and 7cbc3f6).
`openapi-spec-validator` requires `jsonschema-spec` that in turn
wants PyYAML = ">=5.1".

Fixes: #179

Signed-off-by: Stanislav Levin <[email protected]>

Author: stanislavlevin

pyproject.toml

@@ -47,7 +47,6 @@ classifiers = [
 jsonschema = "^4.0.0"
 openapi-schema-validator = "^0.3.2"
 python = "^3.7.0"
-PyYAML = ">=5.1"
 requests = {version = "*", optional = true}
 importlib-resources = "^5.8.0"
 jsonschema-spec = "^0.1.1"