Update CI image

The former one does not support Python 3.9.

Author: Hugo Osvaldo Barrera

.travis.yml

@@ -5,7 +5,7 @@
     ]
   },
   "cache": "pip",
-  "dist": "bionic",
+  "dist": "focal",
   "git": {
     "submodules": false
   },

scripts/make_travisconf.py

@@ -7,7 +7,7 @@
 cfg = {}
 
 cfg['sudo'] = True
-cfg['dist'] = 'bionic'
+cfg['dist'] = 'focal'
 cfg['language'] = 'python'
 cfg['cache'] = 'pip'