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

tests/test-flagprocessor.t

@@ -204,7 +204,8 b' Ensure the data got to the server OK'
     File "*/mercurial/extensions.py", line *, in _runextsetup (glob)
       extsetup(ui)
     File "*/tests/flagprocessorext.py", line *, in extsetup (glob)
-      REVIDX_NOOP, (noopdonothingread, noopdonothing, validatehash,)
+      flagutil.addflagprocessor( (py38 !)
+      REVIDX_NOOP, (noopdonothingread, noopdonothing, validatehash,) (no-py38 !)
     File "*/mercurial/revlogutils/flagutil.py", line *, in addflagprocessor (glob)
       insertflagprocessor(flag, processor, flagprocessors)
     File "*/mercurial/revlogutils/flagutil.py", line *, in insertflagprocessor (glob)