requirements.txt updated

Author: Arun S Kumar

requirements.txt

@@ -5,12 +5,13 @@ flake8-per-file-ignores==0.6
 future==0.18.2
 graphene>=2.1.8,<3
 iso8601==0.1.13
-mongoengine==0.19.1; python_version < '3.0'
-mongoengine==0.20.0; python_version > '3.0'
+mongoengine==0.19.1; python_version < '3.4'
+mongoengine==0.20.0; python_version > '3.4'
 mongomock==3.21.0
 pymongo==3.11.0
 pytest==4.6.11; python_version < '3.0'
 pytest==6.1.2; python_version > '3.0'
+pytest-cov==2.10.1
 singledispatch==3.4.0.3
 # https://stackoverflow.com/a/58189684/9041712
 attrs==20.2.0