lit::shtest-format.py: Make write-bad-encoding.py py3-aware.

  Traceback (most recent call last):
    File "llvm/utils/lit/tests/Inputs/shtest-format/external_shell/write-bad-encoding.py", line 5, in <module>
      sys.stdout.write(b"a line with bad encoding: \xc2.")

sys.stdout.write doesn't accept bytes but sys.stdout.buffer.write accepts.

llvm-svn: 309473