[Python] Fix garbage collection error with Mixture objects (Cantera#2091)

Using shared_ptr route ensures ThermoPhase objects used by MultiPhase
are still present when the MultiPhase destructor runs.

Author: speth

interfaces/cython/cantera/mixture.pxd

@@ -10,7 +10,7 @@ cdef extern from "cantera/equil/MultiPhase.h" namespace "Cantera":
     cdef cppclass CxxThermoPhase "Cantera::ThermoPhase"
     cdef cppclass CxxMultiPhase "Cantera::MultiPhase":
         CxxMultiPhase()
-        void addPhase(CxxThermoPhase*, double) except +translate_exception
+        void addPhase(shared_ptr[CxxThermoPhase], double) except +translate_exception
         void init() except +translate_exception
         void updatePhases() except +translate_exception
 

interfaces/cython/cantera/mixture.pyx

@@ -57,7 +57,7 @@ cdef class Mixture:
 
         for phase,moles in phases:
             # Block species from being added to the phase as long as this object exists
-            self.mix.addPhase(phase.thermo, moles)
+            self.mix.addPhase(phase._base.get().thermo(), moles)
             self._phases.append(phase)
 
         self.mix.init()