Simplify environment handling in srctree tests.

runtests.py

@@ -1525,13 +1525,14 @@ class EndToEndTest(unittest.TestCase):
             .replace("CYTHON", "PYTHON %s" % os.path.join(self.cython_root, 'cython.py'))
             .replace("PYTHON", sys.executable))
         old_path = os.environ.get('PYTHONPATH')
-        os.environ['PYTHONPATH'] = self.cython_syspath + os.pathsep + (old_path or '')
-        try:
+        env = dict(os.environ)
+        env['PYTHONPATH'] = self.cython_syspath + os.pathsep + (old_path or '')
         for command in filter(None, commands.splitlines()):
             p = subprocess.Popen(command,
                                  stderr=subprocess.PIPE,
                                  stdout=subprocess.PIPE,
-                                     shell=True)
+                                 shell=True,
+                                 env=env)
             out, err = p.communicate()
             res = p.returncode
             if res != 0:
@@ -1539,11 +1540,6 @@ class EndToEndTest(unittest.TestCase):
                 print(self._try_decode(out))
                 print(self._try_decode(err))
             self.assertEqual(0, res, "non-zero exit status")
-        finally:
-            if old_path:
-                os.environ['PYTHONPATH'] = old_path
-            else:
-                del os.environ['PYTHONPATH']
         self.success = True