syntax warning of literal

**Describe the bug**
command:
```
./runtests.sh --clean; python -W error -c "import monai"
```
outcome:
```py
  File "/Users/Documents/MONAI/monai/transforms/spatial/array.py", line 176
    if spatial_size is not -1 and spatial_size is not None:
       ^
SyntaxError: "is not" with a literal. Did you mean "!="?
```