Addition of GIL EXIT/ENTER pairs are:
- modos: release the GIL during system calls. CPython does this as well.
- moduselect: release the GIL during the poll() syscall. This call can be
blocking, so it is important to allow other threads to run at this time.
- modusocket: release the GIL during system calls. Many of these calls can
be blocking, so it is important to allow other threads to run.
- unix_mphal: release the GIL during the read and write syscalls in
mp_hal_stdin_rx_chr and mp_hal_stdout_tx_strn. If we don't do this
threads are blocked when the REPL or the builtin input function are used.
- file, main, mpconfigport.h: release GIL during syscalls in built-in
functions that could block.
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