Add libpython3-dev CI dependency

[azeey]

🦟 Bug fix

Summary

#2187 introduced cmake and compiler warnings as a result of removing libdart dependencies:

# cmake
-- Searching for Python - not found.
CMake Warning at /usr/share/cmake/gz-cmake3/cmake3/GzConfigureBuild.cmake:68 (message):
   CONFIGURATION WARNINGS:
   -- Python is missing: Python interfaces are disabled.
Call Stack (most recent call first):
  CMakeLists.txt:238 (gz_configure_build)

and

# gcc
/home/jenkins/workspace/gz_sim-ci-gz-sim8-jammy-amd64/gz-sim/src/SimulationRunner.cc:615:29: warning: unused variable ‘release’ [-Wunused-variable]
  615 |       MaybeGilScopedRelease release;

This adds a direct dependency on libpython3-dev as opposed to the indirect dependency we had through libdart-dev. This PR also fixes the compiler warning in case python bindings are disabled.

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 #2233 (34c21d0) into gz-sim8 (fdbda58) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 34c21d0 differs from pull request most recent head 32419aa. Consider uploading reports for the commit 32419aa to get more accurate results

@@             Coverage Diff             @@
##           gz-sim8    #2233      +/-   ##
===========================================
+ Coverage    65.66%   65.68%   +0.01%     
===========================================
  Files          324      324              
  Lines        30945    30944       -1     
===========================================
+ Hits         20320    20325       +5     
+ Misses       10625    10619       -6     

Files	Coverage Δ
src/SimulationRunner.cc	91.73% <ø> (+0.91%)	⬆️

... and 1 file with indirect coverage changes