Clean up unclosed file handles

Close all file handles that are left dangling to avoid warnings such as

```
ResourceWarning: unclosed file <_io.TextIOWrapper
name='...' mode='r' encoding='UTF-8'> params = json_load(filename.open('r'))
```

tests/optimize/test_hyperopt_tools.py

@@ -209,7 +209,8 @@ def test_export_params(tmpdir):
 
     assert filename.is_file()
 
-    content = rapidjson.load(filename.open('r'))
+    with filename.open('r') as f:
+        content = rapidjson.load(f)
     assert content['strategy_name'] == 'StrategyTestV2'
     assert 'params' in content
     assert "buy" in content["params"]