pyqgis - Making automatic updated date and time field in QGIS

I wish to create a field which automatically updates with the current timestamp when edits are made to a feature.

I am using QGIS 3.4 and I am testing this using a shapefile.

Following @Joseph's answer for this question I tried updating his script to work in QGIS 3.4 as per his instructions (if I followed them correctly, not guaranteed!) to this:

layer = qgis.utils.iface.activeLayer()

def update():
    field = layer.fields().lookupField('fieldName')
    e = QgsExpression( " $now " )
    e.prepare( layer.pendingFields() )
    for feat in layer.selectedFeatures():
        feat[field] = e.evaluate( feat )
        layer.updateFeature( feat )


layer.attributeValueChanged.connect(update)

However this returns the following error:

2018-11-29T11:13:00     WARNING    Traceback (most recent call last):
              File "", line 4, in update
             AttributeError: 'QgsVectorLayer' object has no attribute 'pendingFields'

What's wrong here?

Also will this work on geometry edits or only with attribute edits?

Will this also work for a SpatiaLite layer?