The python3-pip package does not exist on Ubuntu 12.04 LTS.

Try to go via python3-setuptools instead.

.travis.yml

@@ -6,8 +6,10 @@ rust:
 addons:
   apt:
     packages:
-      - python3-pip
+      - python3-setuptools
-install: pip3 install --user --upgrade mypy flake8
+install:
+  - easy_install3 --user pip
+  - python3 -m pip install --user --upgrade mypy flake8
 script: ./test-all.sh
 cache:
   - cargo