src/tests/__init__.py: fix CommandLineTests on python3

src/tests/__init__.py

@@ -435,11 +435,19 @@ def tiff_header_for_ccitt(width, height, img_size, ccitt_group=4):
 
 class CommandLineTests(unittest.TestCase):
     def test_main_help(self):
+        if PY3:
+            from contextlib import redirect_stdout
+            f = StringIO()
+            with redirect_stdout(f):
+                try:
+                    img2pdf.main(['img2pdf', '--help'])
+                except SystemExit:
+                    pass
+            res = f.getvalue()
+            self.assertIn('img2pdf', res)
+        else:
             # silence output
             sys_stdout = sys.stdout
-        if PY3:
-            sys.stdout = TextIOWrapper(BytesIO())
-        else:
             sys.stdout = BytesIO()
 
             try: