`torch.jit.script` is often broken in Python-3.9

Following example:
```
import torch
def foo(x: torch.Tensor) -> float: 
    return x[-3]
bar=torch.jit.script(foo)
print(bar(torch.rand((10,))))
```
fails in Python-3.9 with
```
torch.jit.frontend.NotSupportedError: ellipsis is not supported:
  File "/home/conda/jit.py", line 3
def foo(x: torch.Tensor) -> int:
    return x[-3]
           ~ <--- HERE
```

cc @gmagogsfm