Add explicit copy for pandas>=2.1.0

In Pandas 2.1.0 and later, there are several copy-on-write improvements
that cause beaking changes of the original copying mechanism, see:

    https://pandas.pydata.org/docs/dev/whatsnew/v2.1.0.html#copy-on-write-improvements

This commit ensures that the `_events` of leaf SummaryReader nodes will
be copied before constructing the full dataframe, preventing
involuntarily changing the data inside the `_events` dataframe.

tbparse/summary_reader.py

@@ -267,7 +267,7 @@ def get_events(self, event_type: str) -> pd.DataFrame:
         dfs = []
         if os.path.isfile(self.log_path):
             # Leaf node appends events directly
-            dfs.append(self._events[event_type])
+            dfs.append(self._events[event_type].copy())
         else:
             # Non-leaf node collects children's events
             for child in self._children.values():