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.