Add cryptography dependency for MySQL 8 auth

pymysql needs the cryptography package to speak MySQL 8's default
caching_sha2_password, so 'flask db upgrade' against a stock MySQL 8
failed with 'cryptography package is required'. Make it a real
dependency (dev, prod, CI all connect cleanly) and drop the CI
native-auth workaround that stood in for it.

requirements.in

@@ -11,6 +11,7 @@ marshmallow-sqlalchemy>=0.29
 # Database
 mysql-connector-python>=8.0
 pymysql>=1.1
+cryptography>=42.0   # pymysql needs it for MySQL 8 caching_sha2_password auth
 
 # CLI and utilities
 click>=8.1