dispatch: convert exception payload to bytes more carefully We were previously depending on str() doing something reasonable here, and we can't depend on the objects in question supporting __bytes__, so we work around the lack of direct bytes formatting.

help: convert flag default to bytes portably We were relying on %s using repr on (for example) integer values. Work around that for Python 3 while preserving all the prior magic.

cmdutil: use isstdiofilename() where appropriate