Clear PoolManager in ‘remote_connection’ to ensure sockets are closed

A fix for: #6878

When the ‘keep alive’ option is enabled the remote_connection object will create a PoolManager, but never deliberately clear it.  This can cause ResourceWarning warnings from open sockets.

Added a new "close()" function which explicitly clears the pool.  This close function is called from Webdriver's “quit()” function (+1 squashed commit)

py/selenium/webdriver/remote/remote_connection.py

@@ -440,3 +440,10 @@ def _request(self, method, url, body=None):
         finally:
             LOGGER.debug("Finished Request")
             resp.close()
+
+    def close(self):
+        """
+        Clean up resources when finished with the remote_connection
+        """
+        if hasattr(self, '_conn'):
+            self._conn.clear()

py/selenium/webdriver/remote/webdriver.py

@@ -740,6 +740,7 @@ def quit(self):
             self.execute(Command.QUIT)
         finally:
             self.stop_client()
+            self.command_executor.close()
 
     @property
     def current_window_handle(self):