contrib/automation/requirements.txt.in

py3: conditionalize test-flagprocessor.t on Python 3.8

For reasons I don't understand, Python 3.8 is outputting a different
lint in the traceback than prior Pythons.

The lines in question are:

    flagutil.addflagprocessor(
        REVIDX_NOOP, (noopdonothingread, noopdonothing, validatehash,)
    )

Python <3.8 prints the 2nd line but 3.8 the first line. Perhaps Python
changed its traceback logic to always print the first line of a
multiple line expression?

Whatever the case, with this change, the test now passes on
Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D7945

boto3
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