Delphi: Необходимый пакет, не найденный

В вашем примере я не вижу, как вы вызываете метод toolTip(). Попробуйте это:

import sys
from PyQt5.QtCore    import Qt, QPointF
from PyQt5.QtWidgets import (QGraphicsView, QGraphicsScene, QApplication, 
                             QGraphicsEllipseItem, QGraphicsSceneHoverEvent, 
                             QGraphicsSceneMouseEvent)

class MovableDisk(QGraphicsEllipseItem):
    def __init__(self, top_left_x, top_left_y, radius):
        super().__init__(0, 0, radius, radius) 
        self.setPos(top_left_x, top_left_y)  
        self.setBrush(Qt.red)
        self.setAcceptHoverEvents(True)  

        self.setToolTip("Test")                                          # < +++++ 

    def hoverEnterEvent(self, event: 'QGraphicsSceneHoverEvent'):
        QApplication.instance().setOverrideCursor(Qt.OpenHandCursor)

    def hoverLeaveEvent(self, event: 'QGraphicsSceneHoverEvent'):
        QApplication.instance().restoreOverrideCursor()

    def mouseMoveEvent(self, event: 'QGraphicsSceneMouseEvent'):
        new_cursor_position   = event.scenePos()    
        old_cursor_position   = event.lastScenePos()
        old_top_left_corner   = self.scenePos()
        new_top_left_corner_x = new_cursor_position.x() - old_cursor_position.x() + old_top_left_corner.x()
        new_top_left_corner_y = new_cursor_position.y() - old_cursor_position.y() + old_top_left_corner.y()
        self.setPos(QPointF(new_top_left_corner_x, new_top_left_corner_y))  

    def mousePressEvent(self, event: 'QGraphicsSceneMouseEvent'): pass

    def mouseDoubleClickEvent(self, event: 'QGraphicsSceneMouseEvent'): pass

    def mouseReleaseEvent(self, event: 'QGraphicsSceneMouseEvent'): 
        self.setToolTip("
pos: 
x({}), y({})
"                  # < +++++
                        "".format(self.pos().x(), self.pos().y()))        # < +++++


class MyView(QGraphicsView):
    def __init__(self):
        super().__init__()
        self.scene = QGraphicsScene()
        self.setScene(self.scene)
        self.setSceneRect(0, 0, 250, 250)
        self.disk = MovableDisk(50, 50, 20)
        self.scene.addItem(self.disk)


if __name__ == '__main__':
    app = QApplication([])
    f = MyView()
    f.show()
    sys.exit(app.exec_())