QGIS 3 Get coordinates from mouse click

In a plugin (displayed "Always On Top"), with PyQGIS 3, to avoid it jumps right to the end without calling the display_point function you need following changes in your code:

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from PyQt5.QtCore import Qt

from qgis.gui import QgsMapToolEmitPoint
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    def __init__(self, iface):
        """Constructor.
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        # Create the dialog (after translation) and keep reference
        self.dlg = WhereAmIDialog()
        self.dlg.setWindowFlags(Qt.WindowStaysOnTopHint)
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    def initGui(self):
        """Create the menu entries and toolbar icons inside the QGIS GUI."""
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        result = self.pointTool.canvasClicked.connect(self.display_point)
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    def display_point(self, point, button):  
        # report map coordinates from a canvas click
        coords = "{}, {}".format(point.x(), point.y())
        self.dlg.lineEdit.setText(str(coords)) 
        self.dlg.show()
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    def run(self):
        """Run method that performs all the real work"""
        # make our clickTool the tool that we'll use for now    
        self.canvas.setMapTool(self.pointTool)
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It works; as it can be observed at following image: