tracemalloc: _compare_grouped_stats mutates old_group, and a typo in _group_by

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.

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

Reproduction for (1):

from tracemalloc import Statistic, _compare_grouped_stats
old = {"a": Statistic("a", 100, 2), "b": Statistic("b", 50, 1)}
new = {"a": Statistic("a", 200, 3), "c": Statistic("c", 10, 1)}
old_copy = dict(old)
_compare_grouped_stats(old, new)
print(len(old), len(old_copy))   # 1 2

Linked PRs

• gh-158233
• gh-158371