Update a lot of the intro text in PEP 3156.
PEP 454: the traceback idea comes from the faulthandler module
PEP 454

* add GroupedStats.traceback_limit attribute
* add a Statistic class
* add GroupedStats.statistics() method
* Grouped.compare_to() first parameter is now mandatory, the list is now sorted
  by default, add a new optional sort parameter
* "-X tracemalloc=NFRAME" command line option and PYTHONTRACEMALLOC=NFRAME
  environment variable now specify the number of frames. "-X tracemalloc" is
  still accepted (NFRAME=1)
* rename clear_traces() to reset()
* rename Snapshot.top_by(group_by) to Snapshot.group_by(key_type);
  rename GroupedStats.group_by to GroupedStats.key_type
* get_tracemalloc_memory() only returns a int (total size),
  instead of a (size: int, free: int) tuple
* mention id() builtin function
* rename get_object_trace() to get_object_traceback(), the function now
  only returns the traceback (no more the size)
* rename add_include_filter() to add_inclusive_filter(),
  and add_exclude_filter() to add_exclusive_filter()
* remove the StatsDiff class
* Remove match*() methods from the Filter class
* remove metrics

More details on filename pattern, traces, etc.
PEP 454: copy missing part of the doc
Remove openat support