Custom symbols with invisible linestyle are broken in python 3.9.7, pyqtgraph 0.12.2, PyQt5

[eldond]

Describe the bug
The old method of implementing additional symbols beyond those normally defined in pyqtgraph does not work with a newer python version and newer packages.

To Reproduce

import pgmpl as mpl
import pgmpl.pyplot as plt

x = [1, 2, 3]
y = [4, 5, 6]
plt.gca().plot(x, y, marker='|', ls=' ')

if __name__ == '__main__':
    import sys
    import pgmpl
    from pyqtgraph import QtCore
    app = pgmpl.app  # QtGui.QApplication(sys.argv)
    # Start Qt event loop unless running in interactive mode or using pyside.
    if (app is not None) and ((sys.flags.interactive != 1) or not hasattr(QtCore, 'PYQT_VERSION')):
        app.exec_()

Expected behavior
The symbols should be visible, as they are if the linestyle is visible

Screenshots

The example will work properly again if the symbol isn't one of the custom ones that were added by pgmpl or if the linestyle isn't ' '.

[eldond]

It's just that the marker edge linestyle is inheriting the same linestyle as the line itself, which isn't what we want.