Remove upper version limit for sqlalchemy in `opencensus-ext-sqlalchemy`

[paul-ciobanu]

Describe your environment. Remove upper version limit for sqlalchemy in opencensus-ext-sqlalchemy

        'SQLAlchemy >= 1.1.14, < 1.3.24',  # https://github.com/sqlalchemy/sqlalchemy/issues/6168 # noqa: E501

since https://github.com/sqlalchemy/sqlalchemy/issues/6168 has been fixed this is preventing me to use the latest SQLAlchemy version 1.4.39 in my project