Parsing JSON in templates doesn't work in python3

Hi,
I had to downgrade to python2 in order to make this works:

```
Host: jsonplaceholder.typicode.com
Origin: https://jsonplaceholder.typicode.com

TEMPLATE tryit
myvalue: ${json["try"]}

POST /posts | tryit
{ "try": "it" }
```

with python3 I always had:
```
myvalue: None
```
generated.