修复urllib3的写法，兼容高版本和低版本

Author: yikefan

volcenginesdkcore/rest.py

@@ -35,11 +35,11 @@ def __init__(self, resp):
 
     def getheaders(self):
         """Returns a dictionary of the response headers."""
-        return self.urllib3_response.getheaders()
+        return self.urllib3_response.headers
 
     def getheader(self, name, default=None):
         """Returns a given response header."""
-        return self.urllib3_response.getheader(name, default)
+        return self.urllib3_response.headers.get(name, default)
 
 
 def log_request(method, url, query_params, headers, body, post_params, request_time_out):