[Cython] Bug in 0.24.0a0: bytes passed instead of strings

Hi, when trying to debug cython3 modules from gdb build also with python3, you get a error in gdb plugin code "Python Exception <class 'TypeError'> must be str, not bytes:". This is because in Cython.Debugger.libcython during "cy import" or "cy break" a bytes object is passed to gdb.string_to_argv, but it expects a normal str. I attached a simple fix that worked for me, but I don't know enough about the gdb's plugin system whether it will hold in all cases. The gdb version I ran into this with is 7.10.1.