message: Update the PYTHON-HOWTO.md to include Python 3.9.
  already mentions Python 3.9. So we should also add it in PYTHON-HOWTO.md.
diff: @@ -19,9 +19,11 @@ Currently, the following set of binary wheels are published:
  * Linux
  * Python 3.7
  * Python 3.8
  + * Python 3.9
  * macOS
  * Python 3.7
  * Python 3.8
  + * Python 3.9
  In addition to the binary wheels, a source distribution is also published. If
  `pip` does not find a suitable binary wheel for your environment, it will fall
  