Describe the bug
Trying to download a certificate with the az certificate download command, on azure cli 2.16 and python 3.9, crashes with the error module 'base64' has no attribute 'encodestring'

To Reproduce

• Have python 3.9 and azure cli 2.16

• Have a keyvault mykeyvault with a certificate mycert

• az keyvault certificate download -n mycert --vault-name mykeyvault --file mycert.crt

Expected behavior

The certificate should be downloaded and stored as mycert.txt

Environment summary

• Install Method: dnf package manager with the https://packages.microsoft.com/yumrepos/azure-cli yum repo

• CLI Version:

  $ az --version
  azure-cli                         2.16.0
  
  core                              2.16.0
  telemetry                          1.0.6
  
  Extensions:
  azure-devops                      0.18.0
  
  Python location '/usr/bin/python3'
  Extensions directory '/home/cfosli/.azure/cliextensions'
  
  Python (Linux) 3.9.0 (default, Oct  6 2020, 00:00:00) 
  [GCC 10.2.1 20200826 (Red Hat 10.2.1-3)]
  
  Legal docs and information: aka.ms/AzureCliLegal
  
  Your CLI is up-to-date.

• OS version: Fedora Linux 33 (which comes with Python 3.9 preinstalled)

• Shell type: Bash

Additional context

The Python 3.9 release notes seem to describe the reason and fix quite well:

base64.encodestring() and base64.decodestring(), aliases deprecated since Python 3.1, have been removed: use
base64.encodebytes() and base64.decodebytes() instead. (Contributed by Victor Stinner in bpo-39351.)