py3: don't depend on map() iterating over its input
map(some_generator()) in py2 returns a list, while in py3 it returns a
generator, so the passed-in generator won't be called unless the
returned one is.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null