Basic PyQt5 application to run in ArcGIS Pro custom script tool: running in the thread of Pro

arcgispro_basic_app.py

import sys
import arcpy
from PyQt5.QtWidgets import (QMainWindow, QLabel, QApplication)
from PyQt5.QtCore import Qt


class MainWindow(QMainWindow):
    def __init__(self, *args, **kwargs):
        super(MainWindow, self).__init__(*args, **kwargs)
        self.setWindowTitle("Basic App")

        project = arcpy.mp.ArcGISProject('current')
        label = QLabel("The document version of the open project is {}".format(
            project.documentVersion))
        label.setAlignment(Qt.AlignCenter)
        self.setCentralWidget(label)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = MainWindow()
    ex.show()
    sys.exit(app.exec_())