title: Subtracting two pandas timezone aware timestamps gives TypeError
body: <p>I have two timezone aware pandas datetime timestamps. When I subtract them I get the error: </p>
  
  <pre><code>TypeError: Timestamp subtraction must have the same timezones or no timezones
  </code></pre>
  
  <p>The first is from a conversion to pandas datetime:</p>
  
  <pre><code>start_time = pd.to_datetime(dt.datetime(2019, 6, 28, 0, 52, 57, tzinfo=dt.timezone.utc))
  </code></pre>
  
  <p>The second is from a DataFrame index which looks like this:</p>
  
  <pre><code>&gt;&gt; phase_df.index[0]
  Timestamp('2019-06-28 00:52:30.000130+0000', tz='UTC')
  </code></pre>
  
  <p>To confirm they are both of the same type:</p>
  
  <pre><code>&gt;&gt; type(phase_df.index[0]), type(start_time)
  (pandas._libs.tslibs.timestamps.Timestamp,
   pandas._libs.tslibs.timestamps.Timestamp)
  </code></pre>