STY: Fix whitespace error from new flake8

This only occurs on `v3.5.x` because the test has been rewritten on
`main`.

Author: QuLogic

lib/matplotlib/tests/test_figure.py

@@ -581,7 +581,7 @@ def test_invalid_layouts():
     with pytest.warns(UserWarning):
         # this should warn,
         fig.subplots_adjust(top=0.8)
-    assert not(fig.get_constrained_layout())
+    assert not fig.get_constrained_layout()
 
     # Using layout + (tight|constrained)_layout warns, but the former takes
     # precedence.