Fix regex in Python server model code (#2314)

* - Fix regex in Python server model code.

* - Adding (regular expression) `pattern` to user name and category name.

* - Update Python server related generated code samples.

samples/server/openapi3/petstore/python-flask-python2/openapi_server/models/category.py

@@ -6,8 +6,10 @@ from datetime import date, datetime  # noqa: F401
 from typing import List, Dict  # noqa: F401
 
 from openapi_server.models.base_model_ import Model
+import re
 from openapi_server import util
 
+import re  # noqa: E501
 
 class Category(Model):
     """NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
@@ -86,5 +88,7 @@ class Category(Model):
         :param name: The name of this Category.
         :type name: str
         """
+        if name is not None and not re.search(r'^[a-zA-Z0-9]+[a-zA-Z0-9\.\-_]*[a-zA-Z0-9]+$', name):  # noqa: E501
+            raise ValueError("Invalid value for `name`, must be a follow pattern or equal to `/^[a-zA-Z0-9]+[a-zA-Z0-9\.\-_]*[a-zA-Z0-9]+$/`")  # noqa: E501
 
         self._name = name

samples/server/openapi3/petstore/python-flask-python2/openapi_server/openapi/openapi.yaml

@@ -433,6 +433,7 @@ paths:
         name: username
         required: true
         schema:
+          pattern: ^[a-zA-Z0-9]+[a-zA-Z0-9\.\-_]*[a-zA-Z0-9]+$
           type: string
         style: form
       - description: The password for login in clear text
@@ -641,6 +642,7 @@ components:
           format: int64
           type: integer
         name:
+          pattern: ^[a-zA-Z0-9]+[a-zA-Z0-9\.\-_]*[a-zA-Z0-9]+$
           type: string
       title: Pet category
       type: object