refactor: request_url() to _request_url()

Have all the methods external to the class have matching naming
convention

tradedangerous/plugins/eddblink_plug.py

@@ -31,7 +31,7 @@
 CONTEXT=ssl.create_default_context(cafile=certifi.where())
 
 
-def request_url(url, headers=None):
+def _request_url(url, headers=None):
     data = None
     if headers:
         data = bytes(json.dumps(headers), encoding="utf-8")
@@ -196,7 +196,7 @@ def downloadFile(self, path):
         """
 
         def openURL(url):
-            return request_url(url, headers = {'User-Agent': 'Trade-Dangerous'})
+            return _request_url(url, headers = {'User-Agent': 'Trade-Dangerous'})
 
         if path not in (self.liveListingsPath, self.listingsPath):
             localPath = Path(self.tdb.dataPath, path)