tooling: Fix update sources script for Python 3

This one small change allows the update_glslang_sources.py script to operate correctly with Python 2 and Python 3.

Change the string literal type to "bytes" so that it matches the type returned by the subprocess calls.  Otherwise, under Python 3, the search for "known-good" in the list of remotes always fails.  This is OK for the first execution of update_glsang_sources, since the remote is not there on the first run.  But on subsequent runs, the search still fails to match and the script stops when trying to create a remote that already exists.