[3.9] bpo-40630: Add tracemalloc.reset_peak (GH-20102) (GH-20545)

* bpo-40630: Add tracemalloc.reset_peak (GH-20102, cherrypick 8b626448)

The reset_peak function sets the peak memory size to the current size,
representing a resetting of that metric. This allows for recording the
peak of specific sections of code, ignoring other code that may have
had a higher peak (since the most recent `tracemalloc.start()` or
tracemalloc.clear_traces()` call).

* Adjust docs to point to 3.9