Ensure Linux UCS2 always uses Py2.7.15 (UCS2)

It was not enough to install 2.7.15 and use Pyenv global. The 2.7.13 (UCS2) interpreter was still being recognized.

.travis.yml

@@ -400,6 +400,9 @@ py27_linux_build_wheels_ucs2: &py27_linux_build_wheels_ucs2
   name: "Build wheels - Linux and cp27m (UCS2)"
   env:
     - *base_build_wheels_env
+    # While we install 2.7.15 with UCS2, the Centos6 image already has 2.7.13. To disambiguate
+    # what `python2` resolves to, we require exactly 2.7.15.
+    - PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython==2.7.15']"
     - CACHE_NAME=linuxwheelsbuild.ucs2
   script:
     - docker build --rm -t travis_ci_py27_ucs2

build-support/docker/travis_ci_py27_ucs2/Dockerfile

@@ -8,9 +8,9 @@
 FROM pantsbuild/centos6:latest
 
 # Note we use 2.7.15, rather than 2.7.13, as the centos6 image already comes with 2.7.13
-# installed, which uses UCS4 instead of UCS2. By using 2.7.15, `ci.sh` will set the interpreter
-# constraints to `CPython==2.7.15` when bootstrapping the PEX, which will disambiguate the
-# interpreter version and ensure we always use 2.7.15 with UCS2. 
+# installed, which uses UCS4 instead of UCS2. We pair this change with setting
+# PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS to exactly 2.7.15 in `.travis.yml` to disambiguate
+# the two and always choose the UCS2 interpeter.
 ARG PYTHON_2_VERSION=2.7.15
 RUN yum install sqlite-devel -y
 ENV PYENV_ROOT /pyenv-docker-build

build-support/travis/travis.yml.mustache

@@ -379,6 +379,9 @@ py27_linux_build_wheels_ucs2: &py27_linux_build_wheels_ucs2
   name: "Build wheels - Linux and cp27m (UCS2)"
   env:
     - *base_build_wheels_env
+    # While we install 2.7.15 with UCS2, the Centos6 image already has 2.7.13. To disambiguate
+    # what `python2` resolves to, we require exactly 2.7.15.
+    - PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython==2.7.15']"
     - CACHE_NAME=linuxwheelsbuild.ucs2
   script:
     - docker build --rm -t travis_ci_py27_ucs2