env.fork() raises TypeError: "cannot convert dictionary update sequence element #0 to a sequence"

## 🐛 Bug

`env.reward.spaces` is not deep copy-able. To reproduce:

```py
import compiler_gym
from copy import deepcopy

with compiler_gym.make('llvm-v0') as env:
    deepcopy(env.reward.spaces["IrInstructionCount"])
```