Disable Final for 3.6 (#509)

* Disable Final for 3.6

* dont run mypy for 3.6

.github/workflows/mypy.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.7, 3.8]
 
     steps:
     - uses: actions/checkout@v1

torchani/aev.py

@@ -2,7 +2,15 @@ import torch
 from torch import Tensor
 import math
 from typing import Tuple, Optional, NamedTuple
-from torch.jit import Final
+import sys
+
+if sys.version_info[:2] < (3, 7):
+    class FakeFinal:
+        def __getitem__(self, x):
+            return x
+    Final = FakeFinal()
+else:
+    from torch.jit import Final
 
 
 class SpeciesAEV(NamedTuple):