Fails to parse key names containing single quotes

[Hunga1]

Hello! I'm trying to parse an OpenAPI specification written in YAML that contains a dictionary with key names that are enclosed in single quotes, but the openapi.Parse function is returning the following error:

2022/07/31 22:24:21 [16:20] unexpected key name
      13 |         - args:
      14 |             field: registration
      15 |             mapping:
    > 16 |               '0': init
      17 |               '1': confirmation
                              ^
      18 |               '2': active
exit status 1

Here is the OpenAPI specification that I'm using to produce the error:

info:
  description: Example API
  title: Example API
  version: 1.0.0
openapi: 3.0.1
paths:
  /users:
    description: Users
    get:
      description: Retrieve all users
      operationId: ListUsers
      x-custom-properties:
        - args:
            field: registration
            mapping:
              '0': init
              '1': confirmation
              '2': active

As far as I know, the YAML is valid syntax and to OpenAPI spec.