Use simpler tox.ini, add testing for Python 3.12, and other small updates

[rly]

Motivation

1. We have to update tox.ini regularly when we want to test with new versions of python or make other changes to the testing infastructure. This PR simplifies tox.ini and makes tox more efficient in some ways, though it makes it more complex in others. I think this will be easier to maintain, since the only changes we need to make when adding new versions of python are to the list of predefined environments (which technically do not need to be listed at all, but it is nice to).

2. Add testing for Python 3.12. Fix [Feature]: Add tests for Python 3.12 #904.

3. Replace the styfle/[email protected] action with the github action concurrency key that the workflow authors recommend.

4. Change some links to absolute links in README.rst to work on the PyPI page.

5. Update package versions in environment-ros3.yml

6. Normally all warnings generated from test_gallery.py result in errors / test failures. I realize now that this is too strict for DeprecationWarnings that come from updates to upstream packages. We should inspect those warnings when running local tests, but adding an exception for every DeprecationWarning from an upstream package that we have no control over is very tedious.

How to test the behavior?

Run GitHub Actions workflows

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (44d0d21) 88.65% compared to head (71704fc) 88.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #988   +/-   ##
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  Coverage   88.65%   88.65%           
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  Files          45       45           
  Lines        9451     9451           
  Branches     2688     2688           
=======================================
  Hits         8379     8379           
  Misses        757      757           
  Partials      315      315           

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