MNT: Don't call set_start_method more than once

funpack/main.py

@@ -118,7 +118,8 @@ def main(argv=None):
         # - see the "macOS issues" note in
         # README, and the python bug report:
         # https://bugs.python.org/issue33725
-        mp.set_start_method('fork')
+        if mp.get_start_method(True) is None:
+            mp.set_start_method('fork')
 
         # The icd10 module maintains information
         # which is potentially shared (read from