Merge pull request #4560 from janezd/is-with-literal

manifold.py: Remove 'is' with literal

Author: janezd

Orange/projection/manifold.py

@@ -135,7 +135,7 @@ def __call__(self, data):
             _X, Y, domain = data.X, data.Y, data.domain
             X = dist_matrix = self._metric(_X)
             dissimilarity = 'precomputed'
-        elif self._metric is 'precomputed':
+        elif self._metric == 'precomputed':
             dist_matrix, Y, domain = data, None, None
             X = dist_matrix
             dissimilarity = 'precomputed'