Use PYTHONIOENCODING to enable utf-8 stdout for the nsjail pipe, and handle the potential case where this is bypassable

Since snekbox does not run with a tty, stdout is technically raw bytes, and thus incomplete surrogate pairs can be printed without the client application erroring, and instead fail within _consume_stdout when we attempt to decode it to a str.

This commit sets the PYTHONIOENCODING environment variable to inform python to open the pipe in utf-8 mode.

However, clever use of execl and os.unsetenv() can unset this environment variable, so we add a safety check to _consume_stdout to fail out of parsing output if it contains invalid unicode. This should only happen in deliberate cases, or significant bugs in python or a c library where output is printed to stdout ignoring the python stdout encoding.

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