py3: port test-batching.py to python3
I used byteify-strings.py on this file, then manually added the
various pycompat and bprint bits as needed.
Differential Revision: https://phab.mercurial-scm.org/D5678
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# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL