Depends on gir1.2-handy-0.0, which is not in Ubuntu 18.04

Same here, running Mint 19.3(64). After updating ..
apt list --upgradable -a
Listing... Done
calendar-indicator/bionic,bionic 0.4.0-0extras19.04.1 all [upgradable from: 0.3.4-0extras15.10.0]
calendar-indicator/now 0.3.4-0extras15.10.0 all [installed,upgradable to: 0.4.0-0extras19.04.1]

sudo apt install calendar-indicator
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:
 calendar-indicator : Depends: gir1.2-handy-0.0 but it is not installable
                      Depends: yaru-theme-gtk but it is not installable
E: Unable to correct problems, you have held broken packages.

inxi
System: Host: dm4-HP-Pavilion-dm4-Notebook-PC Kernel: 4.15.0-74-generic x86_64 bits: 64 compiler: gcc v: 7.4.0
           Desktop: MATE 1.22.2 Distro: Linux Mint 19.3 Tricia base: Ubuntu 18.04 bionic

Same thing, I can't upgrade to 20.04 because of this.

 $ sudo do-release-upgrade
proxy 'false' looks invalid
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.

 $ sudo apt upgrade calendar-indicator
...

The following packages have unmet dependencies:
 calendar-indicator : Depends: gir1.2-handy-0.0 but it is not installable
                      Depends: yaru-theme-gtk but it is not installable
E: Broken packages

 $ inxi
CPU~8 core Intel Core i7-7820X (-MT-MCP-) speed/max~1514/4500 MHz Kernel~4.15.0-153-generic x86_64 Up~2:52 Mem~9891.4/31657.3MB HDD~3000.6GB(88.9% used) Procs~576 Client~Shell inxi~2.3.56