[1.8.x] Fixed #24717 -- Fixed model_regress test failure on RHEL6 SCL

The test failed on RHEL6 systems running python 2.7 from a RedHat
Software Collection (SCL) because this test runs an external python
script with a stripped system environment. RedHat SCLs work by setting
a number of system environment variables when these are stripped out by
this test the python 2.7 interpreter is no longer able to function
properly because it can not find the system libraries needed.

Now we use use mock to modify the system environment directly.

Backport of 31e6c9c8e3 from master

tests/model_regress/test_pickle.py

@@ -7,7 +7,8 @@ import warnings
 
 from django.core.files.temp import NamedTemporaryFile
 from django.db import DJANGO_VERSION_PICKLE_KEY, models
-from django.test import TestCase
+from django.test import TestCase, mock
+from django.utils._os import upath
 from django.utils.encoding import force_text
 from django.utils.version import get_version
 
@@ -84,19 +85,16 @@ print(article.headline)"""
         with NamedTemporaryFile(mode='w+', suffix=".py") as script:
             script.write(script_template % pickle.dumps(a))
             script.flush()
-            pythonpath = [os.path.dirname(script.name)] + sys.path
-            env = {
-                # Needed to run test outside of tests directory
-                str('PYTHONPATH'): os.pathsep.join(pythonpath),
-                # Needed on Windows because http://bugs.python.org/issue8557
-                str('PATH'): os.environ['PATH'],
-                str('TMPDIR'): os.environ['TMPDIR'],
-                str('LANG'): os.environ.get('LANG', ''),
-            }
-            if 'SYSTEMROOT' in os.environ:  # Windows http://bugs.python.org/issue20614
-                env[str('SYSTEMROOT')] = os.environ['SYSTEMROOT']
-            try:
-                result = subprocess.check_output([sys.executable, script.name], env=env)
-            except subprocess.CalledProcessError:
-                self.fail("Unable to reload model pickled data")
+            # A path to model_regress must be set in PYTHONPATH
+            model_regress_dir = os.path.dirname(upath(__file__))
+            model_regress_path = os.path.abspath(model_regress_dir)
+            tests_path = os.path.split(model_regress_path)[0]
+            pythonpath = os.environ.get('PYTHONPATH', '')
+            pythonpath = os.pathsep.join([tests_path, pythonpath])
+
+            with mock.patch.dict('os.environ', {'PYTHONPATH': pythonpath}):
+                try:
+                    result = subprocess.check_output([sys.executable, script.name])
+                except subprocess.CalledProcessError:
+                    self.fail("Unable to reload model pickled data")
         self.assertEqual(result.strip().decode(), "Some object")