FIX integer column names for older sqlalchemy version GH6340, GH7330

Author: jorisvandenbossche

pandas/io/sql.py

@@ -567,7 +567,7 @@ def insert(self):
         ins = self.insert_statement()
         data_list = []
         temp = self.insert_data()
-        keys = temp.columns
+        keys = list(map(str, temp.columns))
 
         for t in temp.itertuples():
             data = dict((k, self.maybe_asscalar(v))