`py`: migrate to free-threading python

[jqnatividad]

Currently, because of GIL limitations - the py command is single-threaded and processes in chunks of 50,000 rows.

Now that free-threading is available in Python 3.13 and 3.14, migrate py command to use free-threading for increased performance.

This will necessarily mean dropping qsv support for Python 3.12 and below.

[jqnatividad]

Python 3.13 free-threading is experimental. On 3.14, it's not in the default build.

Only do this once free-threading is no longer experimental and is enabled in the default Python build.