Fix CI (#339)

Replacing Ubuntu 16.04 (which has Python 2.7 and 3.5, which are not
supported by Certbot anymore) by Ubuntu 20.04. Also replacing
venv3 → venv.

Author: felixfontein

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 
 sudo: true
-dist: xenial
+dist: focal
 addons:
   hosts:
     - example.letsencrypt.org
@@ -18,6 +18,7 @@ services:
   - docker
 
 before_install:
+  - sudo apt-get install python3-venv
   - git clone --single-branch --depth=1 -b master https://github.com/certbot/certbot
   - cd certbot
   - ./certbot-auto --os-packages-only -n

appveyor.yml

@@ -17,8 +17,8 @@ artifacts:
 install:
   - git clone --single-branch --depth=1 -b master https://github.com/certbot/certbot
   - cd certbot
-  - python tools\venv3.py
-  - venv3\Scripts\activate.bat
+  - python tools\venv.py
+  - venv\Scripts\activate.bat
   - cd ..
 
 before_build: