Add Flake8 to requirements
Add Flake8 to requirements

Flake8 is used by the Python CI job to verify code style. Add it to
requirements.txt so developers can check their code locally.

Change-Id: Idf6820d85f7692cdae064278dc4987f0dc898a3e

SAGROUP2/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,wikimedia/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,dbrant/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,wikimedia/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,wikimedia/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia