Build: remove pip packages from the install step;

Pip packages were installed because of the meta build in Python, but
they're now unused, so it's a waste of build time and cache space.

.travis.yml

@@ -21,12 +21,6 @@ matrix:
     fast_finish: true
 dist: xenial
 sudo: false
-addons:
-    apt:
-        packages:
-            - python3-pip
-install:
-    - pip3 install --verbose --user --upgrade mypy flake8
 before_script:
     # If an old version of rustfmt from cargo is already installed, uninstall
     # it, since it can prevent the installation of the new version from rustup.
@@ -47,5 +41,3 @@ before_script:
 script: ./test-all.sh
 cache:
     cargo: true
-    directories:
-        - $HOME/.cache/pip