requirements.txt: Set pyOpenSSL version to 16.2.0
requirements.txt: Set pyOpenSSL version to 16.2.0

This fixes development environments with OpenSSL 1.1.0

privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea