flake8: abandon .travis/flake8.sh

given the amount of changes we've recently made, it makes no sense to
copare against the arbitrary 0.0.0 tag we made, just for this script.

Instead we install flake8-mypy, and replace our test with a single call
to flake8.

Author: igalic

.travis.yml

@@ -5,7 +5,6 @@ sudo: false
 env:
   - MYPYPATH="$TRAVIS_BUILD_DIR/.travis/mypy-stubs"
 install:
-  - pip3 install flake8-mutable flake8-builtins mypy
+  - pip3 install flake8-mutable flake8-builtins flake8-mypy
 script:
-  - ".travis/flake8.sh"
-  - mypy iocage
+  - flake8 --exclude=__init__.py --ignore=E203,W391