[qa] Pin down flake8 and isort in extra_requires['qa']

We will use openwisp-utils in all the other modules
to pin down the flake8 and isort versions to reduce
the maintenance overhead of having to update the
flake8 and isort versions in each OpenWISP 2 module

.travis.yml

@@ -30,7 +30,7 @@ before_install:
 install:
   - pip install https://github.com/openwisp/openwisp-users/tarball/master
   - pip install $DJANGO
-  - pip install -e .[users]
+  - pip install -e .[users] .[qa]
 
 before_script:
   - ./runflake8

requirements-test.txt

@@ -1,10 +1,8 @@
 coverage
 coveralls
-isort
-flake8
 # For testing Dependency loaders
 django_netjsonconfig
 djangorestframework>=3.8.2,<3.9
 # TODO: To be removed when we drop python 2.7 support
 # Mock is a standard library from python3.3-pre onwards
-mock>=2.0.0,<2.1.0
+mock>=2.0.0,<2.1.0

setup.py

@@ -39,7 +39,10 @@
     include_package_data=True,
     zip_safe=False,
     install_requires=['django-model-utils>=3.1.2,<3.3.0'],
-    extras_require={'users': ['openwisp-users<0.2']},
+    extras_require={
+        'users': ['openwisp-users<0.2'],
+        'qa': ['flake8<=3.0.4', 'isort<=4.3.4']
+    },
     classifiers=[
         'Development Status :: 3 - Alpha',
         'Environment :: Web Environment',