Fix build script and test failures with python3 build

[kxl-adsk]

Build script was failing to run when executed with python3:

Traceback (most recent call last):
  File "build.py", line 602, in <module>
    BuildAndInstall(context, context.buildArgs, context.stagesArgs)
  File "build.py", line 369, in BuildAndInstall
    RunCMake(context, extraArgs, stagesArgs)
  File "build.py", line 275, in RunCMake
    extraArgs=(" ".join(extraArgs) if extraArgs else "")))
  File "build.py", line 148, in Run
    logfile.write(l.encode("utf8"))
TypeError: write() argument must be str, not bytes

Similarly, a new regression test testMayaUsdLayerEditorCommands.MayaUsdLayerEditorCommandsTestCase was failing:

3: ======================================================================
3: ERROR: testSubLayerEditing (testMayaUsdLayerEditorCommands.MayaUsdLayerEditorCommandsTestCase)
3: test 'mayaUsdLayerEditor' command
3: ----------------------------------------------------------------------
3: Traceback (most recent call last):
3:   File "/Users/ligenzk/Dev/usd/_workspace/Maya_USD/build/RelWithDebInfo/test/lib/testMayaUsdLayerEditorCommands.py", line 203, in testSubLayerEditing
3:     testUsdFile.write(str(DUMMY_FILE_TEXT))
3:   File "/Users/ligenzk/Dev/maya/builds/main/maya/build/RelWithDebInfo/runTime/Maya.app/Contents/Frameworks/Python.framework/Versions/Current/lib/python37.zip/tempfile.py", line 481, in func_wrapper
3:     return func(*args, **kwargs)
3: TypeError: a bytes-like object is required, not 'str'
3: 
3: ----------------------------------------------------------------------

In both cases, we either open the stream in wrong mode or pass bytes when expecting a string.