Description: Don't crash on clean

Author: Rebecca N. Palmer <rebecca_palmer@zoho.com>
Forwarded: no

--- a/setup.py
+++ b/setup.py
@@ -152,14 +152,14 @@ class CleanCommand(Command):
                     ".orig",
                 ):
                     self._clean_me.append(filepath)
-            self._clean_trees.append(pjoin(root, d) for d in dirs if d == "__pycache__")
+            self._clean_trees.extend([pjoin(root, d) for d in dirs if d == "__pycache__"])
 
         # clean the generated pxi files
         for pxifile in _pxifiles:
             pxifile_replaced = pxifile.replace(".pxi.in", ".pxi")
             self._clean_me.append(pxifile_replaced)
 
-        self._clean_trees.append(d for d in ("build", "dist") if os.path.exists(d))
+        self._clean_trees.extend([d for d in ("build", "dist") if os.path.exists(d)])
 
     def finalize_options(self) -> None:
         pass