add testing for the pantsd/pantsd-runner logic path to ensure we can write fatal exceptions to a log in py3

[cosmicexplorer]

See this comment on #7073.

In #7073, we have to change sys.stderr to sys.stderr.buffer, along with changing print() to a .write() in order to write bytes to stderr. We need to do this elsewhere, noted in the comment linked above, and these are all on the pantsd/pantsd-runner logic path. We should:

• validate that the stream we receive as an argument to ExceptionSink.reset_interactive_output_stream() is ready to receive bytes (e.g. changing sys.stderr to sys.stderr.buffer) this should actually be done with type hints, see Add type hints to codebase #6742
• add testing for the pantsd/pantsd-runner logic path to ensure we can write fatal exceptions to a log in python 3 land

See #7220 for a discussion of related pantsd testing we really need to add sooner rather than later.

[stuhood]

Possibly invalidated by #7390.