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The callback PyOS_ReadlineFunctionPointer (used to read a line from the standard input) must return a buffer allocated by PyMem_Malloc(), but PyOS_Readline() releases the GIL before calling PyOS_ReadlineFunctionPointer.
Simplified extract of PyOS_Readline():
 Py_BEGIN_ALLOW_THREADS
 if (!isatty (fileno (sys_stdin)) || !isatty (fileno (sys_stdout)))
 rv = PyOS_StdioReadline (sys_stdin, sys_stdout, prompt);
 else
 rv = (*PyOS_ReadlineFunctionPointer)(sys_stdin, sys_stdout,
 prompt);
 Py_END_ALLOW_THREADS
tok_nextc() calls PyOS_Readline() and calls PyMem_FREE() to release its result.
PyOS_ReadlineFunctionPointer should allocate memory using malloc(), not using PyMem_Malloc(). But PyOS_Readline() should copy the line into a buffer allocated by PyMem_Malloc() to keep backward compatibility.
See also issue #18203 and #3329.