python3: fix python initialization

vtkPythonInterpreter::SetupPythonPrefix calls Py_SetProgramName to guide the initialization of Python. However, it called a few Py_Get.. functions which caused Python3 config to get partially initialized using values prior to calling the Py_SetProgramName. This was resutling in packages failing to init Python correctly with an embedded Python package. Fixed that.

backport: paraview/release