snippet for def classmethod

https://github.com/Microsoft/vscode-python/blob/b741e1af6d53cdf94765a6708964d489bd558ff8/snippets/python.json#L146-L153

should be

``` py
"body": [ 
    "@classmethod",
    "def ${1:funcname}(cls, ${2:parameter_list}):", 
    "\t${3:pass}" 
]
```