Update dependency cryptography to v3 (#259)

Co-authored-by: Renovate Bot <[email protected]>

Author: renovate[bot]

requirements.txt

@@ -7,7 +7,7 @@ psycopg2==2.8.5
 jsonschema==3.2.0
 celery==4.4.5
 redis==3.5.3
-cryptography==2.9.2
+cryptography==3.0
 markdown2==2.3.9
 twilio==6.44.1
 pyumetric==0.0.4