[AIRFLOW-3257] Pin flake8 version to avoid checks changing over time (#4092)

Flake8 3.6.0 was just released and it introduced some new checks that
didn't exist before. As a result all of our CI pipelines are now failing.

To avoid this happening in future we should pin the version of flake8
to 3.5.0 (currently this is in tox.ini, and setup.py)

Author: sprzedwojski

scripts/ci/requirements.txt

@@ -31,7 +31,7 @@ dill
 distributed
 docker-py
 filechunkio
-flake8
+flake8==3.5.0
 flask
 flask-admin
 flask-bcrypt

tox.ini

@@ -81,7 +81,7 @@ commands =
 basepython = python3
 
 deps =
-    flake8
+    flake8==3.5.0
 
 commands =
     {toxinidir}/scripts/ci/flake8_diff.sh