There is only dev-python/pyopenssl-17.5.0 in portage now, but gajim ebuild has following in RDEPEND: >=dev-python/pyopenssl-0.14[${PYTHON_USEDEP}] !>=dev-python/pyopenssl-17.4.0[${PYTHON_USEDEP}] which causes a block.
Thanks. This is fixed in gajim-0.16.9.