test, contrib, refactor: use with when opening a file

test/functional/feature_versionbits_warning.py

@@ -58,7 +58,8 @@ class VersionBitsWarningTest(BitcoinTestFramework):
 
     def versionbits_in_alert_file(self):
         """Test that the versionbits warning has been written to the alert file."""
-        alert_text = open(self.alert_filename, 'r', encoding='utf8').read()
+        with open(self.alert_filename, 'r', encoding='utf8') as f:
+            alert_text = f.read()
         return VB_PATTERN.search(alert_text) is not None
 
     def run_test(self):