fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-CRYPTOGRAPHY-1070544

Author: snyk-bot

requirements.txt

@@ -2,7 +2,7 @@ asn1crypto==0.24.0
 certifi==2019.3.9
 cffi==1.12.3
 chardet==3.0.4
-cryptography==2.6.1
+cryptography==3.3.2
 httpretty==0.9.7
 idna==2.8
 mock==2.0.0