Fix backward compatility

Solution from: Alexis-benoist/eralchemy#80

eralchemy2/sqla.py

@@ -48,7 +48,7 @@ def table_to_intermediary(table):
     """Transform an SQLAlchemy Table object to it's intermediary representation. """
     return Table(
         name=table.fullname,
-        columns=[column_to_intermediary(col) for col in table.c._colset]
+        columns=[column_to_intermediary(col) for col in getattr(table.c,"_colset",getattr(table.c,"_data",{}).values())]
     )