* Enable --gen-onefile in Python
Made it possible to generate all python code in one file. Modified
py_test.py so that it can switch between the multi-file code and
the one-file code.
Updated PythonTest.sh and py_test.py so that the multi-file code
and the one-file code can be tested based on the same test code.
* Sync with google/flatbuffers
* Add --gen-onefile to generate_code.py