pep8, ignore W606 'async' and 'await' are reserved keywords starting with Python 3.7

This error exists in pyjs extension, but pyjs isn't going to Python3.
'noqa' isn't working for W606, so this warning is disabled completely.
'async' and 'await' are still checked by pylint.

#! gmake

yml := ../../yml2
ysl2files := $(wildcard *.ysl2)
xsltfiles := $(patsubst %.ysl2, %.xslt, $(ysl2files))

all:$(xsltfiles)

%.xslt: %.ysl2 yslt_noindent.yml2
	$(yml)/yml2c -I $(yml) $< -o $@.tmp
	xmlstarlet fo $@.tmp > $@
	rm $@.tmp
	
clean:
	rm -f $(xsltfiles)