Does not work with prompt-toolkit 3.0.9

Installing `faculty-sync` from scratch will pull prompt-toolkit 3.0.9, which leads to the following failure when trying to start `faculty-sync`:
```
Traceback (most recent call last):
File "/Users/victor/.local/bin/faculty-sync", line 5, in
from faculty_sync import run
File "/Users/victor/.local/pipx/venvs/faculty-sync/lib/python3.7/site-packages/faculty_sync/__init__.py", line 1, in
from .app import run # noqa
File "/Users/victor/.local/pipx/venvs/faculty-sync/lib/python3.7/site-packages/faculty_sync/app.py", line 8, in
from .ui import View
File "/Users/victor/.local/pipx/venvs/faculty-sync/lib/python3.7/site-packages/faculty_sync/ui.py", line 11, in
from prompt_toolkit.eventloop import get_event_loop
ImportError: cannot import name 'get_event_loop' from 'prompt_toolkit.eventloop' (/Users/victor/.local/pipx/venvs/faculty-sync/lib/python3.7/site-packages/prompt_toolkit/eventloop/__init__.py)
```
Downgrading `prompt-toolkit` to 2.0.9 fixes it.

Might be worth reconsidering #14 for stability.