[df] Register Hist() pythonization only if available

If ROOT is built without the root7 option, the method doesn't exist
which leads to errors when registering the pythonization.

Fixes commit 295345b ("[df] Pythonize Hist() with list of axes")

(cherry picked from commit c30f543)

Author: hahnjo

bindings/pyroot/pythonizations/python/ROOT/_pythonization/_rdataframe.py

@@ -524,7 +524,8 @@ def pythonize_rdataframe(klass):
         getter = MethodTemplateGetter(getattr(klass, method_name), HistoProfileWrapper, model_class)
         setattr(klass, method_name, getter)
 
-    klass.Hist = MethodTemplateGetter(klass.Hist, HistWrapper)
+    if hasattr(klass, "Hist"):
+        klass.Hist = MethodTemplateGetter(klass.Hist, HistWrapper)
 
     klass._OriginalFilter = klass.Filter
     klass._OriginalDefine = klass.Define