Use FindPython3 explicitly instead of FindPythonInterp implicitly

[sloretz]

These packages were using PYTHON_EXECUTABLE, but weren't find_package()ing FindPythonInterp. Upstream PRs ros2/rosidl#612 and ament/ament_cmake#355 removed two uses of PythonInterp. This PR makes it use Python3::Interpreter from FindPython3 because CMake 3.12 deprecates FindPythonInterp.

[pablogs9]

Thanks a lot for this PR @sloretz!

[codecov-commenter]

Codecov Report

Merging #34 (d28b558) into main (2ad69f9) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main     #34   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          1       1           
  Lines          9       9           
  Branches       2       2           
=====================================
  Misses         9       9           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2ad69f9...d28b558. Read the comment docs.