[Cython] By default, use same interpreter for mechanism conversions

Fixes failures when only the Python 3 module is present.

interfaces/cython/cantera/__init__.py

@@ -4,4 +4,9 @@ from .liquidvapor import *
 from .utils import *
 
 import os as _os
+import sys as _sys
 add_directory(_os.path.join(_os.path.dirname(__file__), 'data'))
+
+# Python interpreter used for converting mechanisms
+if 'PYTHON_CMD' not in _os.environ:
+    _os.environ['PYTHON_CMD'] = _sys.executable