Minor fix for mypy check under Python 3.9

Author: jmfernandez

treecript/parser.py

@@ -211,7 +211,7 @@ def metrics_parser(
     pids["command"] = main_commands
     pids["command_label"] = command_labels
     pids["full_command"] = full_command
-    pids["full_stats"] = pd.array(full_stats, dtype=object, copy=False)
+    pids["full_stats"] = pd.array(full_stats, dtype="object", copy=False)
     pids["subtree_root"] = subtree_root
 
     return pids, num_cpu_cores, sampling_period_seconds