ci: fix Python package in coverage workflow

[striezel]

Description

This pull request fixes the E: Package 'python' has no installation candidate error in the coverage workflow by providing an existing package.

The Python package in Ubuntu 22.04 is python3, but I decided to go for python-is-python3 instead, because that makes sure that python3 is installed (as dependency) and also creates a symlink from /usr/bin/python to python3 as a way to make sure that commands with just python (instead of python3) also invoke the installed version of Python 3.

References

See https://github.com/boostorg/gil/actions/runs/7317309558/job/19932692704 for an example of a currently failing coverage job.

Tasklist

• [ ] Ensure all CI builds pass
• [ ] Review and approve

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1df8c24) 81.10% compared to head (8888bf5) 82.12%. Report is 7 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #740 +/- ## =========================================== + Coverage 81.10% 82.12% +1.02% =========================================== Files 117 117 Lines 5171 5355 +184 =========================================== + Hits 4194 4398 +204 + Misses 977 957 -20 ```