PantsRequirement environment markers are broken for 1.10.0.dev5

[jsirois]

Until these two bits of release.sh are updated to unpin pants and its contribs from 2.7:

pants/build-support/bin/release.sh

Lines 63 to 68 in 1ec7de8

	PKG_PANTS=(
	"pantsbuild.pants"
	"//src/python/pants:pants-packaged"
	"pkg_pants_install_test"
	"--python-tag cp27 --plat-name $(find_plat_name)"
	)

pants/build-support/bin/release.sh

Lines 214 to 215 in 1ec7de8

	run_local_pants setup-py \
	--run="bdist_wheel ${BDIST_WHEEL_FLAGS:---python-tag py27}" \

The pants_requirement macro must emit environment markers that constrain the requirement to 2.7 as well:

pants/src/python/pants/backend/python/pants_requirement.py

Line 53 in 1ec7de8

env_marker = "python_version>='3.4' or (python_version>='2.7' and python_version<'3')"