fix slicing for LabelTensor (#167)

* fix slicing for LabelTensor
* Update testing_pr.yml for solving python3.1 error

pina/label_tensor.py

@@ -197,8 +197,22 @@ class LabelTensor(torch.Tensor):
         Return a copy of the selected tensor.
         """
         selected_lt = super(Tensor, self).__getitem__(index)
-        if hasattr(self, 'labels'):
-            selected_lt.labels = self.labels
+
+        try:
+            len_index = len(index)
+        except TypeError:
+            len_index = 1
+
+        if isinstance(index, int) or len_index == 1:
+            if selected_lt.ndim == 1:
+                selected_lt = selected_lt.reshape(1, -1)
+            if hasattr(self, 'labels'):
+                selected_lt.labels = self.labels
+        elif len_index == 2:
+            if selected_lt.ndim == 1:
+                selected_lt = selected_lt.reshape(-1, 1)
+            if hasattr(self, 'labels'):
+                selected_lt.labels = self.labels[index[1]]
 
         return selected_lt