Fix resource warning from patchcheck.py

Tools/scripts/patchcheck.py

@@ -45,6 +45,7 @@ def changed_files():
         sys.exit('need a checkout to get modified files')
 
     st = subprocess.Popen(cmd.split(), stdout=subprocess.PIPE)
+    try:
         st.wait()
         if vcs == 'hg':
             return [x.decode().rstrip() for x in st.stdout]
@@ -52,6 +53,8 @@ def changed_files():
             output = (x.decode().rstrip().rsplit(None, 1)[-1]
                       for x in st.stdout if x[0] in b'AM')
         return set(path for path in output if os.path.isfile(path))
+    finally:
+        st.stdout.close()
 
 
 def report_modified_files(file_paths):