If GIL is disabled then there's threat of a race condition if some other
code specifically requests USB processing (i.e. to unblock stdio), while
a scheduled TinyUSB callback is already running on another thread.
Relies on the change in the parent commit, where scheduler is restricted
to main thread if GIL is disabled.
Fixes#15390 - "TinyUSB callback can't recurse" exceptions on rp2 when
using _thread module and USB serial I/O.
Adds a unit test for stdin functioning correctly in threads (fails on rp2
port without this fix).
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
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