reqirements.txt: made changes

removed the redundant version
of the setuptools from 21.0.0 to 21

Closes coala#5176

requirements.txt

@@ -8,6 +8,6 @@ packaging~=16.8
 Pygments~=2.1
 PyPrint~=0.2.6
 requests~=2.12
-setuptools>=21.0.0
+setuptools>=21
 testfixtures~=5.3.1
 unidiff~=0.5.2