try to reduce memory

Signed-off-by: Jinzhe Zeng <[email protected]>

.github/workflows/test_python.yml

@@ -105,12 +105,12 @@ jobs:
       run: |
         from glob import glob
 
-        contents = []
-        for fn in glob(".test_durations_*"):
-            with open(fn) as f:
-                contents.append(f.read().strip().strip("{}"))
         with open(".test_durations", "w") as f:
-            f.write("{" + ",".join(contents) + "}")
+            f.write("{")
+            for fn in glob(".test_durations_*"):
+                with open(fn) as f:
+                    f.write(f.read().strip().strip("{}"))
+            f.write("}")
       shell: python
 
   pass: