checks: correct the shebang line filtering for python files
As it is, the only related file is docs/test2rst.py, which was covered by **.py.
Not sure if it matters, but most patterns in core tests are for "#!.*?python".
(Though there are a couple "#!.*python" tests.)
#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 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null