Make python code compatible with urllib3 v2.6.0+ (#22520)

openapi-generator still uses methods that have been removed from urllib3
v2.6.0. The solution is as described in urllib3's changelog:

> Removed the HTTPResponse.getheaders() method in favor of
> HTTPResponse.headers. Removed the HTTPResponse.getheader(name,
> default) method in favor of HTTPResponse.headers.get(name, default).
> (#3622)

See https://urllib3.readthedocs.io/en/latest/changelog.html

Close #22514

samples/client/echo_api/python/openapi_client/exceptions.py

@@ -129,7 +129,7 @@ class ApiException(OpenApiException):
                     self.body = http_resp.data.decode('utf-8')
                 except Exception:
                     pass
-            self.headers = http_resp.getheaders()
+            self.headers = http_resp.headers
 
     @classmethod
     def from_response(