21718: Replace .decode method with str(bytes, "utf-8")

21718: Replace .decode method with str(bytes, "utf-8")

This is because some bytes-like objects (such as memoryview) don't
have a "decode()" method. It appears that the preferred way to get a
decoded string is to simply call the constructor with the bytes-like
object and the desired encoding.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>