packages.apt: install libpython3-dev

[scpeters]

🦟 Bug fix

Fixes Ubuntu 20.04 builds on main that fail to find Python

Summary

The Ubuntu 20.04 builds on main fail to find Python since gazebo-tooling/release-tools#709 was merged and the libpython3-dev was no longer installed in our Jenkins Ubuntu builds. This adds the package to our packages.apt file.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #414 (66c5ef6) into main (fb741dd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #414   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files          46       46           
  Lines        4260     4260           
=======================================
  Hits         4256     4256           
  Misses          4        4           

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[methylDragon]

Oh, this was why it was failing during the bumps :o
CI is green, so we should be ok.

Pinging @chapulina to double check.