I don't believe you can access those properties through the standard functionality of arcpy
as the symbologyType property on a raster layer with a stretched symbology returns the unsupported "other".
Update October 2019:
With the release of version 2.4 of ArcPro you can access most of the layer properties via CIM, this includes the symbology. Using the AddIn CIM Viewer you can see the StretchType which indicates this is a property that can be altered via CIM using a few lines of python.
Screen shot from CIM Viewer
This is new functionality exposed in ArcPro, not ArcMap. Be aware this approach by-passes all the inbuilt error checking in arcpy
so you can potentially blitz your project if you are not careful, heed the warn on the ESRI help file!