Revert "TST: Add sqlalchemy to test requirements"

This reverts commit 94532bd826e0c76d0116e92adadc185fcf235aa3.
Revert "ENH: Use sqlalchemy for PostGIS connection for better compatibility with pandas.read_sql"

This reverts commit 75c530115e7ed08fbf7a90ce75a64260bc59e87e.
TST: Temporarily freeze pandas version to before pandas SQL API change