Configure travis' flake8 call more explicitly

I'm basing this on '{posargs:jedi}' looking like it was a tox thing,
which we're no longer using.

.travis.yml

@@ -31,7 +31,7 @@ matrix:
         - 'pip install .[qa]'
       script:
         # Ignore F401, which are unused imports. flake8 is a primitive tool and is sometimes wrong.
-        - 'flake8 --extend-ignore F401 {posargs:jedi}'
+        - 'flake8 --extend-ignore F401 jedi setup.py'
 install:
     - sudo apt-get -y install python3-venv
     - pip install .[testing]