Fix failing test in python 3.7

can't use Magicmock in 3.7 (works in 3.8 though).

freqtrade/exchange/common.py

@@ -110,7 +110,7 @@ def retrier_async(f):
                 count -= 1
                 kwargs.update({'count': count})
                 logger.warning('retrying %s() still for %s times', f.__name__, count)
-                if isinstance(ex, DDosProtection) or isinstance(ex, RetryableOrderError):
+                if isinstance(ex, DDosProtection):
                     await asyncio.sleep(calculate_backoff(count + 1, API_RETRY_COUNT))
                 return await wrapper(*args, **kwargs)
             else:

tests/exchange/test_exchange.py

@@ -62,7 +62,7 @@ def ccxt_exceptionhandlers(mocker, default_conf, api_mock, exchange_name,
 async def async_ccxt_exception(mocker, default_conf, api_mock, fun, mock_ccxt_fun,
                                retries=API_RETRY_COUNT + 1, **kwargs):
 
-    with patch('freqtrade.exchange.common.asyncio.sleep'):
+    with patch('freqtrade.exchange.common.asyncio.sleep', get_mock_coro(None)):
         with pytest.raises(DDosProtection):
             api_mock.__dict__[mock_ccxt_fun] = MagicMock(side_effect=ccxt.DDoSProtection("Dooh"))
             exchange = get_patched_exchange(mocker, default_conf, api_mock)