Bump cryptography to get Python 3.8 wheels

Author: pquentin

dev-requirements.txt

@@ -9,7 +9,7 @@ pytest-random-order==1.0.4;python_version>="3.6"
 pytest-timeout==1.3.3
 pytest-cov==2.7.1
 h11==0.8.0
-cryptography==2.6.1
+cryptography==2.8
 flaky==3.6.1
 
 # https://github.com/GoogleCloudPlatform/python-repo-tools/issues/23