Fill in missing export translator implementation

[csyshing]

Python translator export never worked properly, but the mismatch between the wrapping and the Python implementation is causing errors that we would like to eliminate as a potential source of problems.

Where no export is supported, canExport() and exportObject() methods are not expected to be implemented in Python translators - leaving dummy methods which don't return the corret type will cause errors.

If implementing the method but return the incorrect type, you will get a runtime error like this:

TF_PYTHON_EXCEPTION: in TfPyConvertPythonExceptionToTfErrors at line 114 of ../pxr/base/lib/tf/pyError.cpp -- Tf Python Exception
TypeError: No registered converter was able to produce a C++ rvalue of type AL::usdmaya::fileio::translators::ExportFlag from this Python object of type bool

[csyshing]

The unit test failed for the same reason as #1726 and #1727 on Windows + Maya 2019, seems like a problem on the Jenkins machine? Any idea?

[seando-adsk]

I'll have a look at the failing build. Yesterday we updated all builds to use USD v21.08.