class MainForm(QtGui.QMainWindow, Ui_MainWindow):
    def __init__(self):
        super(MainForm, self).__init__()
        self.setupUi(self)

        self.qgisMapCanvas.mapRenderer().setProjectionsEnabled(True)
        self.canvas_crs=core.QgsCoordinateReferenceSystem(3857)
        assert self.canvas_crs.isValid()
        self.qgisMapCanvas.mapRenderer().setDestinationCrs(self.canvas_crs)
...

Qt: Untested Windows version 10.0 detected!
Running In An Application Bundle
Error in sys.excepthook:
Traceback (most recent call last):
  File "qgisutils.py", line 196, in qgis_excepthook
  File "qgisutils.py", line 107, in showException
  File "qgisutils.py", line 142, in open_stack_dialog
AttributeError: 'NoneType' object has no attribute 'messageBar'

Original exception was:
Traceback (most recent call last):
  File "CallMainWin.py", line 324, in <module>
    win = MainForm()
  File "CallMainWin.py", line 49, in __init__
    assert self.canvas_crs.isValid()
AssertionError
[64268] Failed to execute script CallMainWin

assert self.canvas_crs.isValid()