[python-flask] Fix #19487 SyntaxWarning (#19489)

* Bugfix: #19487: Fix SyntaxWarning

* Fix a bug which generates SyntaxWarning

* Chore: #19487: Generate sample

modules/openapi-generator/src/main/resources/python-flask/model.mustache

@@ -147,7 +147,7 @@ class {{classname}}(Model):
 {{/minimum}}
 {{#pattern}}
         if {{name}} is not None and not re.search(r'{{{vendorExtensions.x-regex}}}', {{name}}{{#vendorExtensions.x-modifiers}}{{#-first}}, flags={{/-first}}re.{{.}}{{^-last}} | {{/-last}}{{/vendorExtensions.x-modifiers}}):  # noqa: E501
-            raise ValueError("Invalid value for `{{name}}`, must be a follow pattern or equal to `{{{pattern}}}`")  # noqa: E501
+            raise ValueError(r"Invalid value for `{{name}}`, must be a follow pattern or equal to `{{{pattern}}}`")  # noqa: E501
 {{/pattern}}
 {{#maxItems}}
         if {{name}} is not None and len({{name}}) > {{maxItems}}:

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

@@ -86,6 +86,6 @@ def name(self, name: str):
         :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
+            raise ValueError(r"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