Slack webhook not working via `ntfy`? Here's a workaround for now

**Problem**
Slack webhook will not work with `ntfy==2.7.0`.

**Workaround**
To get the latest `ntfy` package, please install the package from source
```
$ sudo python -m pip install git+https://github.com/dschep/ntfy.git@master --upgrade
```

**Related Issue**
https://github.com/dschep/ntfy/issues/233