[Extractor] Fix startup error

On Python 3 need to create a copy of the dict to iterate

Fixes: #3264

CHANGELOG.md

@@ -9,6 +9,10 @@
 - Fix request.base 'idna' codec error (#3261).
 - Fix unable to change password (#3262).
 
+### Extractor plugin
+
+- Fix potential error starting plugin.
+
 ### Documentation
 
 - Fix macOS install typo.

deluge/plugins/Extractor/deluge_extractor/core.py

@@ -89,7 +89,7 @@ else:
     # Test command exists and if not, remove.
     for command in required_cmds:
         if not which(command):
-            for k, v in EXTRACT_COMMANDS.items():
+            for k, v in list(EXTRACT_COMMANDS.items()):
                 if command in v[0]:
                     log.warning('%s not found, disabling support for %s', command, k)
                     del EXTRACT_COMMANDS[k]