fix use of is_Texture in plot3d base

Author: fchapoton

src/sage/plot/plot3d/base.pyx

@@ -55,7 +55,7 @@ from sage.modules.free_module_element import vector
 
 from sage.rings.real_double import RDF
 from sage.misc.temporary_file import tmp_filename
-from .texture import Texture, is_Texture
+from .texture import Texture
 from .transform cimport Transformation, point_c, face_c
 include "point_c.pxi"
 
@@ -2673,7 +2673,7 @@ cdef class PrimitiveObject(Graphics3d):
     def __init__(self, **kwds):
         if 'texture' in kwds:
             self.texture = kwds['texture']
-            if not is_Texture(self.texture):
+            if not isinstance(self.texture, Texture):
                 self.texture = Texture(self.texture)
         else:
             self.texture = Texture(kwds)
@@ -2687,7 +2687,7 @@ cdef class PrimitiveObject(Graphics3d):
             sage: G.set_texture(color='yellow'); G
             Graphics3d Object
         """
-        if not is_Texture(texture):
+        if not isinstance(texture, Texture):
             texture = Texture(texture, **kwds)
         self.texture = texture