Add python 3.9 in release CI (#830)

Author: Pahandrovich

.ci/scripts/gen_release_jobs.py

@@ -23,7 +23,7 @@
 args = parser.parse_args()
 
 CHANNELS = args.channels
-PYTHON_VERSIONS = ['3.6', '3.7', '3.8']
+PYTHON_VERSIONS = ['3.7', '3.8', '3.9']
 SYSTEMS = ['ubuntu-latest', 'macos-latest', 'windows-latest']
 ACTIVATE = {
     'ubuntu-latest': 'conda activate',