repr() of deeply nested dicts and dictviews can cause a stack overflow.

When repr() take a deeply nested dicts as the argument, the stack overflows and  Rust Python interpreter crashes.

test.py
```
x = {}
for i in range(10000):
    x = {1: x}

repr(x)

```

Actual output:

> Segmentation fault (core dumped)


System:

> Ubuntu 16.04
> Rust Python 0.1.2 interpreter