README: add PYFLAKES=/usr/bin/pyflakes3 env variable
pyflakes 2.x nowadays complains about aa-genprof:60: undefined name 'PermissionError'
We already deprecated py2 support in 2.11 (and obviously nobody tests with py2 anymore), therefore recommending pyflakes3 makes sense.
Maybe we should also change our Makefiles to use py3 and pyflakes3 by default, but that will be another (master-only) commit.
I propose this patch for 2.11..master