prompt-toolkit hanging

Hi! I'm trying to run prompt-toolkit (the main prompt engine for xonsh shell) and facing with hanging:
```xsh
# rustpython v0.4.0 (https://github.com/RustPython/RustPython#ebdc033c)
rustpython -m pip install prompt-toolkit
```
ptk.py:
```xsh
from prompt_toolkit import prompt
text = prompt("Say something: ")
print("You said:", text)
```
Run:
```xsh
~/.cargo/bin/rustpython ptk.py
# Hanging
```