python3-opencv fails to install due to unmet dependency.

[spec]

1)
Description: Ubuntu 20.04.3 LTS
Release: 20.04

2)
python3-opencv:
  Installed: (none)
  Candidate: 4.2.0+dfsg-5
  Version table:
     4.2.0+dfsg-5 500
        500 http://us.archive.ubuntu.com/ubuntu focal/universe amd64 Packages

3) I expected 'apt install python3-opencv` to complete without issue.

4)
Issuing: 'apt install python3-opencv` fails with:

"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:
 python3-opencv : Depends: libopencv-dnn4.2 (= 4.2.0+dfsg-5+20.04.sav2) but it is not going to be installed
                  Depends: libopencv-superres4.2 (= 4.2.0+dfsg-5+20.04.sav2) but it is not going to be installed
                  Depends: libopencv-contrib4.2 (>= 4.2.0+dfsg) but it is not going to be installed
E: Unable to correct problems, you have held broken packages."

Drilling down, 'libopencv-dnn4.2' requires 'libprotobuf23' which is not available for 20.04, but is available for 20.10.