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/api_client.py

@@ -313,7 +313,7 @@ class ApiClient:
                 return_data = self.__deserialize_file(response_data)
             elif response_type is not None:
                 match = None
-                content_type = response_data.getheader('content-type')
+                content_type = response_data.headers.get('content-type')
                 if content_type is not None:
                     match = re.search(r"charset=([a-zA-Z\-\d]+)[\s;]?", content_type)
                 encoding = match.group(1) if match else "utf-8"
@@ -330,7 +330,7 @@ class ApiClient:
         return ApiResponse(
             status_code = response_data.status,
             data = return_data,
-            headers = response_data.getheaders(),
+            headers = response_data.headers,
             raw_data = response_data.data
         )
 
@@ -702,7 +702,7 @@ class ApiClient:
         os.close(fd)
         os.remove(path)
 
-        content_disposition = response.getheader("Content-Disposition")
+        content_disposition = response.headers.get("Content-Disposition")
         if content_disposition:
             m = re.search(
                 r'filename=[\'"]?([^\'"\s]+)[\'"]?',

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(

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

@@ -49,12 +49,17 @@ class RESTResponse(io.IOBase):
             self.data = self.response.data
         return self.data
 
+    @property
+    def headers(self):
+        """Returns a dictionary of response headers."""
+        return self.response.headers
+
     def getheaders(self):
-        """Returns a dictionary of the response headers."""
+        """Returns a dictionary of the response headers; use ``headers`` instead."""
         return self.response.headers
 
     def getheader(self, name, default=None):
-        """Returns a given response header."""
+        """Returns a given response header; use ``headers.get()`` instead."""
         return self.response.headers.get(name, default)