Find correct python3 version in pybind11-config

[nicolecheetham]

Improves logic so Python3 will not be set by pybind11-config file unless a user has not already set the python version

Resolves #22051

---

This change is 

[nicolecheetham]

+@BetsyMcPhail for feature review please

Reviewable status: LGTM missing from assignee BetsyMcPhail, needs platform reviewer assigned, needs at least two assigned reviewers, missing label for release notes (waiting on @nicolecheetham)

[BetsyMcPhail]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: needs platform reviewer assigned, needs at least two assigned reviewers

[jwnimmer-tri]

I'll take on the platform review here.

[jwnimmer-tri]

The claim in the overview is "Resolves 22051". What evidence do we have to prove it is resolved?

The pre-merge tests do not seem to have any leverage to over the resolution (since no test code or configs were changed here), so in that case it is incumbent upon the PR author to explain in the pull review text somwhere what testing they performed to confirm the fix, presumably either as links to online test runs against this branch, or list(s) of manual commands that were run locally to confirm what happens.

If 22051 had a reproduction recipe clearly outlined then I could try it myself, but I couldn't tease out any reproducer from the posting.

[nicolecheetham]

@jwnimmer-tri I was thinking of procuring evidence by testing the drake-ros repo as Betsy brought to my attention it had the pybind11 issue in the past. Is there a way to locally test the drake-ros repository? Or do I have create an PR to test?

[jwnimmer-tri]

There is run_all_tests.sh but per the TODOs it doesn't actually run everything. (It's also possible that it has bitrotted, since I don't think it has any CI coverage.) But you probably don't need to test everything anyway -- the git repo has a bunch of different modules (kinda like drake-external-examples) and I imagine only one or two of them are relevant to this change. If you can find even one build or test that is failing when run vs Drake master, but passing vs this pull request, that would be sufficient.

Or, it's certainly fine to open a pull request against drake-ros if that is helpful for CI testing. Just mark it as "draft" and comment that it's only for some testing so nobody starts accidentally reviewing it. However, beware that some parts of drake-ros CI are probably known to be broken (unrelated to this Python3 problem), so also beware of false negatives.

In any case, if you had a bug reproducer that didn't involved drake-ros, that would probably be the easiest thing to test. I would hope that some project in drake-external-examples could demonstrate the bug, probably with a few tweaks to its CMakeLists.txt to be able to hit the problem.

[nicolecheetham]

Local testing was performed to confirm the fix.
Steps Taken (tri mac mini 1):

• cd drake-external-examples/drake_cmake_installed

• ./setup/install_prereqs

• ./.github/ci_build_test #Check to make sure tests work properly as is

• Change to home directory:

• cd /opt/drake/lib/cmake/pybind11

• vim pybind11-config.cmake #Made change to find_package for python3

• Change directory back to drake-external-examples/drake_cmake_installed

• ./.github/ci_build_test #Check to see tests still work after change

• Edit drake_cmake_installed Cmake code so find_package(pybind11) command occurred prior find_package(Python3)

• ./.github/ci_build_test #Check to see if tests pass meaning known pybind11-config error was resolved

All three times, the ci_build_test succeeded
Aside: the build_test failed when pybind11 was prior to find_package(Python3) and no edit had been made to the local version of pybind11-config.cmake. This was the expected result

[jwnimmer-tri]

Okay, me say that back to you to make sure I got it.

• You have a list of manual commands to run to test this PR.
• When you run those commands against the master branch, then the example doesn't work correctly.
• When you run those commands against this PR branch, then the example does work correctly.

Assuming I've got that right, then .

[nicolecheetham]

Yes, that is correct

Reviewable status: commits need curation (https://drake.mit.edu/reviewable.html#curated-commits) (waiting on @nicolecheetham)