Missing backend due to outdated pip?

I'm getting this erro:

```
ERROR: Invalid backend slack_webhook
```

with the following config file

```
backends: [slack_webhook]
slack_webhook:
url: "my_url"
user: "Julio"
```

I noticed the latest version on pip is 2.7.0 from Nov 2018. Is the pip package no longer maintained, do I need to install ntfy in a different way?