Hide stderr output from subprocess call in test suite.

anntzer · anntzer · commit 6fd1ea3bf0f5 · 2021-03-30T12:38:34.000+02:00
Otherwise the (irrelevant and slightly confusing) traceback gets
displayed when running `pytest -s`.
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -481,11 +481,11 @@ def test_backend_fallback_headless(tmpdir):
     with pytest.raises(subprocess.CalledProcessError):
         subprocess.run(
             [sys.executable, "-c",
-             ("import matplotlib;" +
-              "matplotlib.use('tkagg');" +
-              "import matplotlib.pyplot")
+             "import matplotlib;"
+             "matplotlib.use('tkagg');"
+             "import matplotlib.pyplot"
              ],
-            env=env, check=True)
+            env=env, check=True, stderr=subprocess.DEVNULL)
 
 
 @pytest.mark.skipif(
