Try using a newer travis dist to fix cryptography building issues

SmileyChris · SmileyChris · commit 3adfaa9ce130 · 2021-03-30T10:56:14.000+13:00
diff --git a/.travis.yml b/.travis.yml
@@ -10,6 +10,7 @@ deploy:
     tags: true
 install: pip install -U tox-travis
 language: python
+dist: focal
 python:
 - 3.9
 - 3.8
