db = QtSql.QSqlDatabase.addDatabase('QMYSQL')
 db.setHostName('localhost')
 db.setUserName('root')
 db.setPassword('1234')
 db.setDatabaseName('test')
 db.open()
 print(db.lastError().text())

 python gui.py
QSqlDatabase: QMYSQL driver not loaded
QSqlDatabase: available drivers: QSQLITE QMYSQL QMYSQL3 QODBC QODBC3 QPSQL QPSQL7
Driver not loaded Driver not loaded

 from PyQt5 import QtWidgets, QtSql
import sys
app = QtWidgets.QApplication(sys.argv)
# (и дальше уже все, как у вас)

 if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    win = LogIn()
    win.show()
    sys.exit(app.exec_())