Install Failure on Ubuntu 20.04

### Steps to reproduce

On a new install of Ubuntu 20.04, following install documentation for ralph, failure occurs at step:
`sudo apt-get install mysql-server nginx ralph-core`

### Expected behavior

Expected successful installation of package.

### Actual behavior

```
$sudo apt-get install ralph-core
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
ralph-core : Depends: libmysqlclient20 but it is not installable
E: Unable to correct problems, you have held broken packages.
```

### Environment

* Ralph version: latest
* Operating system: Ubuntu 20.04
* Method of installation: Deb package, following provided documentation.