fix pythonpath in test_setuptools_support

Author: RonnyPfannschmidt

testing/test_setuptools_support.py

@@ -53,9 +53,15 @@ def makeinstall(version):
 
 
 def check(packagedir, expected_version, **env):
+
+    old_pythonpath = os.environ.get('PYTHONPATH')
+    if old_pythonpath:
+        pythonpath = '{}:{}'.format(old_pythonpath, packagedir)
+    else:
+        pythonpath = str(packagedir)
     subprocess.check_call(
         [sys.executable, "-c", SCRIPT, expected_version],
-        env=dict(os.environ, PYTHONPATH=".:" + str(packagedir), **env),
+        env=dict(os.environ, PYTHONPATH=pythonpath, **env),
     )