Port QtVCP from PyQt5 to QtPy

Replace all PyQt5-specific imports and idioms with their QtPy
equivalents, making the codebase compatible with PyQt5 and PyQt6.

Import changes:
- Replace `from PyQt5.XXX import` with `from qtpy.XXX import`
- Replace bare `from PyQt5 import XXX` with `from qtpy import XXX`
- Replace `pyqtSignal`, `pyqtSlot`, `pyqtProperty` with the qtpy
  canonical names `Signal`, `Slot`, `Property`
- Port `PyQt5.QtDesigner` imports to `qtpy.QtDesigner`
- Port `PyQt5.Qsci` imports to `qtpy.Qsci`

PDFViewer: add QtPdf backend as Qt6 alternative to popplerqt5
- Try `qtpy.QtPdf.QPdfDocument` first; fall back to popplerqt5
- Use `QPdfDocumentRenderOptions` with Annotations flag for render
  quality matching the popplerqt5 Antialiasing/TextAntialiasing hints

sys_notify: make DBus main loop integration backend-agnostic
- Try dbus.mainloop.pyqt5, then dbus.mainloop.pyqt6.

qtplasmac handlers: replace `find_spec("PyQt5.QtWebEngineWidgets")`
- Use a module-level `_WEBENGINE_AVAILABLE` flag set by try/import
  of `qtpy.QtWebEngineWidgets` in the qtplasmac handler.

plugins/status_label_plugin: fix pyqtProperty descriptor check
- Replace fragile `'PyQt5.QtCore.pyqtProperty' in str(...)` with
  `isinstance(..., Property)` to work across all Qt bindings

Author: bgermann

configs/sim/qtvcp_screens/qtaxis_lathe/qtaxis/qtaxis_handler.py

@@ -5,8 +5,8 @@
 import os
 import linuxcnc
 
-from PyQt5 import QtCore, QtWidgets
-from PyQt5.QtGui import QColor
+from qtpy import QtCore, QtWidgets
+from qtpy.QtGui import QColor
 
 from qtvcp.widgets.mdi_line import MDILine as MDI_WIDGET
 from qtvcp.widgets.gcode_editor import GcodeEditor as GCODE

configs/sim/qtvcp_screens/qtdragon/qtvcp/screens/qtdragon/qtdragon_handler.py

@@ -1,5 +1,5 @@
 import os, time
-from PyQt5 import QtCore, QtWidgets, QtGui
+from qtpy import QtCore, QtWidgets, QtGui
 from qtvcp.widgets.gcode_editor import GcodeEditor as GCODE
 from qtvcp.widgets.gcode_graphics import GCodeGraphics as GRAPHICS
 from qtvcp.widgets.mdi_line import MDILine as MDI_WIDGET
@@ -28,7 +28,7 @@
 QHAL = Qhal()
 
 try:
-    from PyQt5.QtWebEngineWidgets import QWebEnginePage
+    from qtpy.QtWebEngineWidgets import QWebEnginePage
 except:
     LOG.warning('QtDragon Warning with loading QtWebEngineWidget - is python3-pyqt5.qtwebengine installed?')
 

configs/sim/qtvcp_screens/qtvcp_custom/qtdefault_ngc/qtdefault_ngc_handler.py

@@ -2,8 +2,8 @@
 # **** IMPORT SECTION **** #
 ############################
 
-from PyQt5 import QtCore
-from PyQt5 import QtWidgets
+from qtpy import QtCore
+from qtpy import QtWidgets
 from qtvcp.widgets.origin_offsetview import OriginOffsetView as OFFVIEW_WIDGET
 from qtvcp.widgets.tool_offsetview import ToolOffsetView as TOOLVIEW_WIDGET
 from qtvcp.widgets.dialog_widget import CamViewDialog as CAMVIEW

configs/sim/qtvcp_screens/qtvcp_experimental/qtdefault_gladevcp/qtdefault_gladevcp_handler.py

@@ -2,7 +2,7 @@
 # **** IMPORT SECTION **** #
 ############################
 
-from PyQt5 import QtCore, QtWidgets
+from qtpy import QtCore, QtWidgets
 from qtvcp.widgets.origin_offsetview import OriginOffsetView as OFFVIEW_WIDGET
 from qtvcp.widgets.dialog_widget import CamViewDialog as CAMVIEW
 from qtvcp.widgets.dialog_widget import MacroTabDialog as LATHEMACRO

configs/sim/qtvcp_screens/qtvcp_experimental/qtdefault_gladevcp_handler.py

@@ -5,7 +5,7 @@
 import os
 import linuxcnc
 
-from PyQt5 import QtCore, QtWidgets
+from qtpy import QtCore, QtWidgets
 
 from qtvcp.widgets.mdi_line import MDILine as MDI_WIDGET
 from qtvcp.widgets.gcode_editor import GcodeEditor as GCODE

configs/sim/qtvcp_screens/qtvcp_experimental/qtdefault_gladevcp_handler_1.py

@@ -2,8 +2,8 @@
 # **** IMPORT SECTION **** #
 ############################
 
-from PyQt5 import QtCore
-from PyQt5 import QtWidgets
+from qtpy import QtCore
+from qtpy import QtWidgets
 from qtvcp.widgets.origin_offsetview import OriginOffsetView as OFFVIEW_WIDGET
 from qtvcp.widgets.tool_offsetview import ToolOffsetView as TOOLVIEW_WIDGET
 from qtvcp.widgets.dialog_widget import CamViewDialog as CAMVIEW

configs/sim/qtvcp_screens/qtvcp_experimental/qtdefault_ngc/qtdefault_ngc_handler.py

@@ -2,8 +2,8 @@
 # **** IMPORT SECTION **** #
 ############################
 
-from PyQt5 import QtCore
-from PyQt5 import QtWidgets
+from qtpy import QtCore
+from qtpy import QtWidgets
 from qtvcp.widgets.origin_offsetview import OriginOffsetView as OFFVIEW_WIDGET
 from qtvcp.widgets.tool_offsetview import ToolOffsetView as TOOLVIEW_WIDGET
 from qtvcp.widgets.dialog_widget import CamViewDialog as CAMVIEW

configs/sim/woodpecker/1280x1024_5axis/woodpecker_handler.py

@@ -2,10 +2,10 @@
 import linuxcnc
 import hal
 import sys
-from PyQt5.QtWidgets import QMessageBox
-from PyQt5 import QtCore, QtWidgets, QtGui, uic
+from qtpy.QtWidgets import QMessageBox
+from qtpy import QtCore, QtWidgets, QtGui, uic
 try:
-    from PyQt5.QtWebKitWidgets import QWebView
+    from qtpy.QtWebKitWidgets import QWebView
 except ImportError:
     raise Exception("Qtvcp error with woodpecker - is package python-pyqt5.qtwebkit installed?")
 from qtvcp.widgets.gcode_editor import GcodeEditor as GCODE

configs/sim/woodpecker/compensate.py

@@ -21,7 +21,7 @@
 import time
 import linuxcnc
 import hal
-from PyQt5 import QtGui, QtWidgets
+from qtpy import QtGui, QtWidgets
 import numpy as np
 import matplotlib.pyplot as plt
 import matplotlib.cm as cm

configs/sim/woodpecker/qtvcp/screens/woodpecker/resources.py

@@ -6,7 +6,7 @@
 #
 # WARNING! All changes made in this file will be lost!
 
-from PyQt5 import QtCore
+from qtpy import QtCore
 
 qt_resource_data = b"\
 \x00\x00\x08\x28\