Thanks guys - I've put all three way to override python in my package.use!!
# package.use
#
# three ways to select python version
>=app-text/asciidoc-8.6.10 python_single_target_python2_7 python_targets_python2_7
dev-util/itstool PYTHON_TARGETS: -* python3_6 PYTHON_SINGLE_TARGET: -* python3_6
dev-libs/libxml2 python_targets_python*
Add to package.use
dev-util/itstool PYTHON_TARGETS: -* python3_6 PYTHON_SINGLE_TARGET: -* python3_6
dev-libs/libxml2 PYTHON_TARGETS: -* python3_6
I don't specify >=< and package version when overriding default USE flags in package.use
Reference: https://wiki.gentoo.org/wiki/Project:Python/PYTHON_TARGETS
https://wiki.gentoo.org/wiki/Handbook:Parts/Working/USE