[subplot] Segmentation fault when calling `plt::subplot(...)`

[acxz]

When calling plt::subplot() the program terminates with a seg fault. See the examples/subplot.cpp for a minimal example.

Also occurs on plt::suptitle()

The particular line is PyObject* args = PyTuple_New(1)
It seems that PyTuple_New()` is giving this seg fault.

Python: 3.8.2

[acxz]

Maybe dup of #63