Use better temp file for onlyif_unicode_paths test.

IPython/testing/decorators.py

@@ -1,3 +1,4 b''
+# -*- coding: utf-8 -*-
 """Decorators for labeling test objects.
 
 Decorators that merely return a modified version of the original function
@@ -323,11 +324,12 b' null_deco = lambda f: f'
 # Some tests only run where we can use unicode paths. Note that we can't just
 # check os.path.supports_unicode_filenames, which is always False on Linux.
 try:
-    tempfile.mkdtemp(u"€")
+    f = tempfile.NamedTemporaryFile(prefix=u"tmp€")
 except UnicodeEncodeError:
     unicode_paths = False
 else:
     unicode_paths = True
+    f.close()
 
 onlyif_unicode_paths = onlyif(unicode_paths, ("This test is only applicable "
                                     "where we can use unicode in filenames."))