gh-158234: Fix _compare_grouped_stats mutating old_group and a typo

[Tony123-tech]

Two small issues in Lib/tracemalloc.py:

1. _compare_grouped_stats() mutates its old_group argument by popping
   keys from it. After the call, old_group is missing every key that
   was also in new_group. The current caller (Snapshot.compare_to)
   does not reuse the dict, so the bug is latent, but the function
   should not modify its arguments. Fix: copy the dict at the top.

2. A typo in the error message raised by _group_by():
   "cumulative mode cannot by used" should be
   "cumulative mode cannot be used".

No NEWS entry needed: neither change is user-visible.

• Issue: tracemalloc: _compare_grouped_stats mutates old_group, and a typo in _group_by #158234

[bedevere-app]

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[Tony123-tech]

No NEWS entry needed: neither change is user-visible. The
_compare_grouped_stats fix affects a private function, and the typo
fix only changes an error message.

[Tony123-tech]

The typo fix is user-visible only in the sense that the error message
changes. I don't think a NEWS entry is needed. Let me know if you
disagree.