[`devel`]→[`master`] Preparation of v1.3.1

[diegoferigo]

• [x] Resolve conflicts merging master into devel, if needed. db069ff
• [x] Merge devel into master, making sure that everything works as expected.
• [x] Bump the ScenarIO version in the top-level CMakeLists.txt.
• [x] Make sure that scenario.__init__.py defines the correct specifier set for the Ignition distribution.
• [x] Check if any dependency under our control has to be bumped, or if any dependency version has to be updated in setup.py, setup.cfg, and pyproject.toml.
• [x] Create a new GitHub pre-release with a new tag v*.rc0[^1] and make sure that is properly deployed to PyPI. This is necessary for the next step.
• [x] Bump the scenario specifier set in the setup.cfg of gym-ignition in master (e.g. ~= 1.3.1.rc for the 1.3.1 release). It makes all gym_ignition packages compatible only with scenario packages that share the same minor version.
• [x] Create a new GitHub release with a new tag v*[^1].
• [x] Make sure that the release was successfully uploaded in PyPI in both sdist and wheel formats.
• [x] Merge master into devel. This merge must contain the commit associated to the release tag in order to reset the automatic numbering of the pre-release packages of the nightly branch. If a post-release has to be published to fix minor errors, master has to be merged again into devel.
• [x] Bump the scenario specifier set in the setup.cfg of gym-ignition in devel by increasing the least significant number (e.g. >= 1.3.2.dev for the 1.3.1 release, or >= 1.3.1post1.dev for the 1.3.1.post0 release). It makes nightly releases incompatible with stable versions, making sure that calling pip install --pre gym_ignition pulls the pre-release also of scenario.

[^1]: Note that the version without the v prefix must comply with PEP440 in this regexp.

[diegoferigo]

I'm not sure why the Stable channel checks targeting current devel fail with:

[Err] [Physics.cc:680] Failed to find plugin [ignition-physics5-dartsim-plugin]. Have you checked the IGN_GAZEBO_PHYSICS_ENGINE_PATH environment variable?

We never modify the IGN_GAZEBO_PHYSICS_ENGINE_PATH env variable. The Stable channel is tested against Ignition Gazebo Fortress installed from ppa, and in this setup all the environment variables should be already taken care from upstream.

[diegoferigo]

After releasing v1.3.0.post0, we did not merge master into devel. The subsequent pre-release versions should have been v.1.3.0.post1.devX, but devel was using >=v1.3.1.dev that overrided the CI scenario installation with the one from PyPI.

The release process was updated recently and there are still some edge cases like this one that have to be properly handled. I updated the release guide with the right specifier version to use.