Setting threads parameter leads to Cbc crash

[Sceriffo]

Describe the bug Setting threads parameter using model's setter method doesn't work and leads to crash of Cbc at the end of computation.

ERROR while running Cbc. Signal SIGSEGV caught. Getting stack trace.
...
python3(_PyObject_MakeTpCall+0x296) [0x5f74f6]
python3(_PyEval_EvalFrameDefault+0x5dc4) [0x571164]
python3(_PyEval_EvalCodeWithName+0x26a) [0x569dba]
python3(_PyFunction_Vectorcall+0x393) [0x5f6eb3]
python3(_PyEval_EvalFrameDefault+0x85a) [0x56bbfa]
python3(_PyEval_EvalCodeWithName+0x26a) [0x569dba]
python3(_PyFunction_Vectorcall+0x393) [0x5f6eb3]
python3(_PyEval_EvalFrameDefault+0x85a) [0x56bbfa]
python3(_PyFunction_Vectorcall+0x1b6) [0x5f6cd6]
python3(_PyEval_EvalFrameDefault+0x72d) [0x56bacd]
python3(_PyEval_EvalCodeWithName+0x26a) [0x569dba]
python3(_PyFunction_Vectorcall+0x393) [0x5f6eb3]
python3(_PyEval_EvalFrameDefault+0x72d) [0x56bacd]
python3(_PyEval_EvalCodeWithName+0x26a) [0x569dba]
python3(PyEval_EvalCode+0x27) [0x6902a7]
python3() [0x67f951]
python3() [0x67f9cf]
python3() [0x67fa71]
python3(PyRun_SimpleFileExFlags+0x197) [0x681b97]
python3(Py_RunMain+0x212) [0x6b9d32]
python3(Py_BytesMain+0x2d) [0x6ba0bd]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf3) [0x7f92f51de083]
python3(_start+0x2e) [0x5fc5fe]

To Reproduce

model.threads = 2
model.optimize()

Expected behavior Threads limited to 2 and Cbc oesn't crash

Desktop (please complete the following information):

• Operating System, version: Ubuntu 20.04.4 LTS
• Python version: 3.8.10
• Python-MIP version (we recommend you to test with the latest version): 1.14.1

Additional context I notice that it works fine by using the Cbc wrapped method. Like:

cbc_set_parameter(model.solver, "threads", "{}".format(2))

[Sceriffo]

I might have a fix.

[sebheger]

@Sceriffo We are more that happy to review a pull request