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"]

tests/strategy/test_strategy_loading.py

@@ -62,8 +62,8 @@ def test_load_strategy(default_conf, result):
 
 
 def test_load_strategy_base64(result, caplog, default_conf):
-    with (Path(__file__).parents[2] / 'freqtrade/templates/sample_strategy.py').open("rb") as file:
-        encoded_string = urlsafe_b64encode(file.read()).decode("utf-8")
+    filepath = Path(__file__).parents[2] / 'freqtrade/templates/sample_strategy.py'
+    encoded_string = urlsafe_b64encode(filepath.read_bytes()).decode("utf-8")
     default_conf.update({'strategy': 'SampleStrategy:{}'.format(encoded_string)})
 
     strategy = StrategyResolver.load_strategy(default_conf)