Pin setuptools to < 4.0

This change pins setuptools to a version <4.0 in requiremetns.txt
in order to get around a bug that has shown up in a setuptools update
today. (Bug 1325423)

This pin will be removed once the bug has been fixed.

Change-Id: Ia7c68091ccb7663ca50745ba80ad23e57c6e8488
Merge "Pin setuptools to < 4.0"