Update sqlalchemy requirement from ~=1.3.20 to ~=1.3.21

Updates the requirements on [sqlalchemy](https://github.com/sqlalchemy/sqlalchemy) to permit the latest version.
- [Release notes](https://github.com/sqlalchemy/sqlalchemy/releases)
- [Changelog](https://github.com/sqlalchemy/sqlalchemy/blob/master/CHANGES)
- [Commits](https://github.com/sqlalchemy/sqlalchemy/commits)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -3,4 +3,4 @@ lxml~=4.6.2
 pytz~=2020.4
 pytest-cov~=2.10.1
 mysql-connector~=2.2.9
-SQLAlchemy~=1.3.20
+SQLAlchemy~=1.3.21