Support comparing two sets of pystats (GH-98816)

This adds support for comparing pystats collected from two different builds.

- The `--json-output` can be used to load in a set of raw stats and output a
  JSON file.
- Two of these JSON files can be provided on the next run, and then comparative
  results between the two are output.

Author: mdboom

Doc/using/configure.rst

@@ -131,7 +131,8 @@ General Options
    Turn on internal statistics gathering.
 
    The statistics will be dumped to a arbitrary (probably unique) file in
-   ``/tmp/py_stats/``, or ``C:\temp\py_stats\`` on Windows.
+   ``/tmp/py_stats/``, or ``C:\temp\py_stats\`` on Windows. If that directory
+   does not exist, results will be printed on stdout.
 
    Use ``Tools/scripts/summarize_stats.py`` to read the stats.