fix: pkg_resources deprecated warning

[SubaruArai]

This pr fixes a warning caused by pkg_resources, by using importlib.metadata instead.

Importlib.metadata is supported since py3.8, which is shipped with the oldest supported ROS1/2 distro (noetic, Ubuntu 20.04)

Same warning in colcon (maybe resolved)

[SubaruArai]

@cottsay I've added importlib_metadata for py3.6.

Couple of questions:

1. should I change stdeb.cfg and bump the minimum OS and python version? Or should I add importlib_metadata to it?
2. should we drop setuptools from the dependency? AFAIK, it was added for pkg_resources. Require setuptools, src/rosdep2/platforms/pip.py imports pkg_resources #809

[SubaruArai]

@cottsay ping

[cottsay]

Good questions.

1. should I change stdeb.cfg and bump the minimum OS and python version? Or should I add importlib_metadata to it?

Should be safe to add python3 (>= 3.8) | python3-importlib-metadata as a dependency to the stdeb.cfg like this.

2. should we drop setuptools from the dependency? AFAIK, it was added for pkg_resources. Require setuptools, src/rosdep2/platforms/pip.py imports pkg_resources #809

Yes, please do. Both setup.cfg and stdeb.cfg reference it.

[SubaruArai]

@cottsay Couple of comments / questions (they'll be in their own pr if they need to be addressed)

1. I left get_version_strings's output as setuptools x.y.z, for backwards compatibility. Or should I change it?
2. stdeb.cfg contains python2 related config. Should we delete it like in colcon-core?
3. the ci file contains python2 tests. Should we delete it?

[cottsay]

Thanks for iterating.

1. I left get_version_strings's output as setuptools x.y.z, for backwards compatibility. Or should I change it?

I think it's good as-is. The setuptools version is useful here not because we're using setuptools (pkg_resources) to get the information, but because at the time, the packages used setuptools to build. While there are more options now and pip supports them, setuptools is still widely used and could be useful information debugging things.

2. stdeb.cfg contains python2 related config. Should we delete it like in colcon-core?
3. the ci file contains python2 tests. Should we delete it?

These need to be separate changes for sure, and I think they should be made prior to merging this change so that we can point to the exact moment we dropped Python 2 support from rosdep. I've started this process in catkin_pkg, and there will likely be a bit more to it than changing ci.yaml and stdeb.cfg. Can you give me a few days to work through that process?

[SubaruArai]

Ping me once it's merged, I'll fix merge conflicts. (if any)

[cottsay]

Can we add a simple test to exercise this code? Something like this could be added to test_rosdep_pip.py:

def test_PipInstaller_get_version_strings():
    from rosdep2.platforms.pip import PipInstaller
    installer = PipInstaller()
    assert installer.get_version_strings()

[SubaruArai]

@cottsay Done!

[cottsay]

Thanks for iterating