force certifi to install only for python3.0 and up

requirements.txt

@@ -67,7 +67,7 @@ boto3==1.7.71 \
 botocore==1.10.71 \
     --hash=sha256:9302ad235db66efa9d11c664b1cb0b259826d82a206446460ea05bcfcc431a4a \
     --hash=sha256:ffa673c9a53f3ab4eba4ce8cf9d736177ca67509827e716cb5070f0b621fb0a7
-certifi==2018.8.13 \
+certifi==2018.8.13; python_version >= '3.0' \
     --hash=sha256:4c1d68a1408dd090d2f3a869aa94c3947cc1d967821d1ed303208c9f41f0f2f4 \
     --hash=sha256:b6e8b28b2b7e771a41ecdd12d4d43262ecab52adebbafa42c77d6b57fb6ad3a4
 cffi==1.11.5 \