Modernize repository configuration

[FrederikSchnack]

Summary

This is the first PR in a sequence of general improvements.

Modernize the repository configuration and testing infrastructure without changing SymPDE’s production code.

Changes

• Replace the legacy combined CI workflow with a dedicated testing workflow.
• Run CI when pull requests are opened, reopened, synchronized, or marked ready for review.
• Modernize project and build metadata in pyproject.toml.
• Define optional dependency groups for testing and documentation.
• Move pytest and coverage configuration into pyproject.toml.
• Update installation and testing instructions in the README.
• Move TODO.rst to old/ – see Look into old/ #195.
• Change README to md.
• Add AUTHORS file.
• Add CHANGELOG.md.

[yguclu]

Good job! I have posted a few comments here and there.

BTW, I feel that this is the right PR to add an AUTHORS file, and probably a CHANGELOG too. What do you think?

[FrederikSchnack]

BTW, I feel that this is the right PR to add an AUTHORS file, and probably a CHANGELOG too. What do you think?

I think that's a good idea! Do you want to add them?

[yguclu]

BTW, I feel that this is the right PR to add an AUTHORS file, and probably a CHANGELOG too. What do you think?

I think that's a good idea! Do you want to add them?

Unfortunately, I do not think I will have the time before the developers' meeting tomorrow. If you want to draft them based on what we have in PSYDAC, I can probably review them

[FrederikSchnack]

Unfortunately, I do not think I will have the time before the developers' meeting tomorrow. If you want to draft them based on what we have in PSYDAC, I can probably review them

Done in 7fde16b and f85db9a. For now, I only wrote Version 0.19.0 to 0.20.0. For consistency I also changed the README to md in b370bf8.

[campospinto]

I had a quick look but it seems good to me, thanks!