title: Tkinter Treeview issue when inserting rows with tags
body: <p>I have just switched from python 3.6 to python 3.7. I have a function which inserts rows in a Treeview tree with tags. The tags are used for giving a foreground color and a background color to the rows inserted to the tree. My code was working ok when I was using python 3.6. Once I switched to 3.7 the rows inserted were not given a background or foreground color but where only given a white background and a black foreground color.</p>
  
  <p>There doesn't seem to be a syntax change in tkinter.ttk from python 3.6 to 3.7 regarding tag configuration or tree insert.</p>
  
  <pre><code>tree.tag_configure('MATCHED', foreground='dark green', background='gray98')
  tree.tag_configure('UNMATCHED', foreground='red2', background='gray98')
  
  if match_status== '1':
      tree.insert('', 'end', text=df_row, values=my_value, tag='MATCHED')
  elif match_status == '0':
      tree.insert('', 'end', text=df_row, values=my_value, tag='UNMATCHED')`
  </code></pre>
  
  <p>It is expected that when the rows are inserted to the tree to be given the correct background and foreground color.</p>
  
  <p>Any help is appreciated.</p>