[BUG][Python] Client generation doesn't handle regex pattern correctly

[aaronatbissell]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating a python client library where a string pattern like this is present, the validation logic in the client code is incorrect

datetime:
  type: string
  pattern: '[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

openapi-generator version

5.4.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: a spec to test free form object models
  version: 1.0.0
  title: OpenAPI Petstore
  license:
    name: Apache-2.0
    url: "https://www.apache.org/licenses/LICENSE-2.0.html"
tags: []
paths: {}
components:
  schemas:
    DateTimeObject:
      properties:
        datetime:
          type: string
          pattern: '[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

Generation Details

openapi-generator generate -i spec.yaml -g python

This generates a validation string of:

validations = {
  ('datetime',): {
    'regex': {
      'pattern': r'[d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z/', # noqa: E501
    },
  },
}

Steps to reproduce

Using a config like this:

openapi: 3.0.0
info:
  description: a spec to test free form object models
  version: 1.0.0
  title: OpenAPI Petstore
  license:
    name: Apache-2.0
    url: "https://www.apache.org/licenses/LICENSE-2.0.html"
tags: []
paths: {}
components:
  schemas:
    DateTimeObject:
      properties:
        datetime:
          type: string
          pattern: '[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

Then running

openapi-generator generate -i spec.yaml -g python

This generates a validation string of:

validations = {
  ('datetime',): {
    'regex': {
      'pattern': r'[d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z/', # noqa: E501
    },
  },
}

and it should be

r'[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

Related issues/PRs

#1823 - related but for the python-server code base
#8098 - similar, but python-flask and specific to escaped characters

Suggest a fix

I'll try to post a PR soon

[aaronatbissell]

I think this was introduced with #10920, merged to fix #6675