2to3 follow-up: stdin happens to be switched in text mode in PY3
This extends and, at least partly, finalizes commit
"2to3: PY3 distinguishes text (abstract type) and data (raw bytes)".
This can be verified, e.g., with 'echo | python3 ./run-dev cmd-wrap',
which would previously fail "TypeError: decoding str is not supported".
Signed-off-by: Jan Pokorný <jpokorny@redhat.com>