Make it possible to collect pystats of parts of runs

[mdboom]

The --enable-pystats system has a couple of important shortcomings:

• When stats is turned off, stats aren't dumped (on quit or explicitly). Therefore, it's impossible to put sys._stats_on/sys._stats_off calls around tested code and get the results out.
• Since stats collection is on by default, it is very fiddly to exclude code for stats, especially for things that fire off subprocesses, e.g. pyperformance.

@markshannon's suggestion is to:

• Having stats off by default
• Add an -Xstats flag to turn it on at startup
• Always dump out stats

Linked PRs

• gh-100143: Improve collecting pystats for parts of runs #100144

[markshannon]

We also need to add a mechanism to always have stats on, for gathering stats for applications which cannot be modified or spawn subprocesses.

Adding an environment variable analog for -Xstats, e.g. PYTHONSTATS should be sufficient.

[hauntsaninja]

Thanks, looks like this is completed