checking out https://github.com/pantsbuild/pants at 4dafc870ece338be7bbba4e44ee1e416e27e9afb, then running test-bisect.zsh, should show that pex==1.5.3 suceeds, and pex==1.6.0 errors out at import time for the google-cloud-bigquery package.

repro-bigquery.diff

diff --git a/3rdparty/python/requirements.txt b/3rdparty/python/requirements.txt
index 9c0e693..0b93f20 100644
--- a/3rdparty/python/requirements.txt
+++ b/3rdparty/python/requirements.txt
@@ -17,7 +17,7 @@ more-itertools<6.0.0 ; python_version<'3'
 packaging==16.8
 parameterized==0.6.1
 pathspec==0.5.9
-pex==1.6.6
+pex==1.5.3
 psutil==5.4.8
 pycodestyle==2.4.0
 pyflakes==2.0.0
diff --git a/examples/3rdparty/python/BUILD b/examples/3rdparty/python/BUILD
index 1cee054..474b97b 100644
--- a/examples/3rdparty/python/BUILD
+++ b/examples/3rdparty/python/BUILD
@@ -4,6 +4,13 @@
 # see/edit requirements.txt in this directory to change deps.
 python_requirements()
 
+python_requirement_library(
+  name='google-cloud-bigquery',
+  requirements=[
+    python_requirement('google-cloud-bigquery==1.10.0'),
+  ],
+)
+
 remote_sources(
   name='tensorflow-framework',
   dest=packaged_native_library,
diff --git a/examples/tests/python/example_test/pkg_resources_access/BUILD b/examples/tests/python/example_test/pkg_resources_access/BUILD
new file mode 100644
index 0000000..bb0d334
--- /dev/null
+++ b/examples/tests/python/example_test/pkg_resources_access/BUILD
@@ -0,0 +1,17 @@
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+python_tests(
+  dependencies=[
+    'tests/python/pants_test:int-test',
+    'examples/3rdparty/python:google-cloud-bigquery',
+  ],
+)
+
+python_binary(
+  name='bin',
+  source='main.py',
+  dependencies=[
+    'examples/3rdparty/python:google-cloud-bigquery',
+  ],
+)
diff --git a/examples/tests/python/example_test/pkg_resources_access/__init__.py b/examples/tests/python/example_test/pkg_resources_access/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/examples/tests/python/example_test/pkg_resources_access/main.py b/examples/tests/python/example_test/pkg_resources_access/main.py
new file mode 100644
index 0000000..e52939c
--- /dev/null
+++ b/examples/tests/python/example_test/pkg_resources_access/main.py
@@ -0,0 +1,10 @@
+# coding=utf-8
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from google.cloud import bigquery
+
+
+print('bigquery, version {}'.format(bigquery.__version__))
diff --git a/examples/tests/python/example_test/pkg_resources_access/test_bigquery_python_run.py b/examples/tests/python/example_test/pkg_resources_access/test_bigquery_python_run.py
new file mode 100644
index 0000000..a749827
--- /dev/null
+++ b/examples/tests/python/example_test/pkg_resources_access/test_bigquery_python_run.py
@@ -0,0 +1,25 @@
+# coding=utf-8
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from google.cloud import bigquery
+from pants_test.pants_run_integration_test import PantsRunIntegrationTest
+
+
+# TODO: ???/explain how you as a user can write pants integration tests, e.g. for binaries you want
+# to make sure work in CI.
+class TestBigqueryPythonRun(PantsRunIntegrationTest):
+
+  # TODO: is this test definitely run in CI?
+  def test_bigquery_python_run(self):
+    pants_run = self.do_command(
+      # "--python-setup-interpreter-constraints=['CPython>=2.7,<3']",
+      'run',
+      'examples/tests/python/example_test/pkg_resources_access:bin',
+    )
+    # Using `-q` in self.do_command() doesn't appear to be making pants run quietly here, but we can
+    # rely on the output containing exactly this line.
+    self.assertIn('\nbigquery, version {}\n'.format(bigquery.__version__),
+                  pants_run.stdout_data)

test-bisect.zsh

#!/usr/bin/env zsh

set -euxo pipefail

export MODE=debug
export PANTS_ENABLE_PANTSD=False

if [[ ! -f 'examples/tests/python/example_test/pkg_resources_access/BUILD' ]]; then
  git apply repro-bigquery.diff
  trap "git checkout -- examples/ && git clean -xfd -- examples/" EXIT
fi

./pants clean-all

./pants test examples/tests/python/example_test/pkg_resources_access -- -vs

exit 0