[cppyy] Minimise direct imports of libcppyy.so.

This will facilitate moving it to a new location in the next commit.

Author: hageboeck

roottest/python/basic/PyROOT_datatypetest.py

@@ -37,9 +37,7 @@ def setup_class(cls):
         cls.datatypes = cppyy.load_reflection_info(cls.test_dct)
         cls.N = cppyy.gbl.N
         # In new Cppyy, nullptr can't be found in gbl.
-        # Take it from libcppyy (we could also use ROOT.nullptr)
-        import libcppyy
-        cls.nullptr = libcppyy.nullptr
+        cls.nullptr = cppyy._backend.nullptr
 
     def test01_load_reflection_cache(self):
         """Loading reflection info twice should result in the same object"""

roottest/python/basic/PyROOT_datatypetest_numpy.py

@@ -37,9 +37,7 @@ def setup_class(cls):
         cls.datatypes = cppyy.load_reflection_info(cls.test_dct)
         cls.N = cppyy.gbl.N
         # In new Cppyy, nullptr can't be found in gbl.
-        # Take it from libcppyy (we could also use ROOT.nullptr)
-        import libcppyy
-        cls.nullptr = libcppyy.nullptr
+        cls.nullptr = cppyy._backend.nullptr
 
     def test01_buffer_to_numpy(self):
         """Wrap buffer with NumPy array"""