Migrate requirements/ to pyproject.toml

[jorgepiloto]

🐞 Problem
The only reason why we have a requirements/ folder is because it was the only way for dependabot to automatically update those. However, the tool supports now pyproject.toml parsing with other build-backends rather than poetry. Since PyMAPDL is using flit, we can migrate all extra dependencies to the Python project file.

💡 How to implement
The following tasks need to be solved:

• Create the following extra groups in the pyproject.toml: doc and tests.
• Update the tox.ini file to use these extra dependencies. Contact @jorgepiloto for more information on Tox migration. Sorry. Just found this project does not use Tox.
• Update the CI to use these extra dependencies.

[jorgepiloto]

Pinging here @germa89, @RobPasMue and @clatapie. Camille may be interested in working in this issue at some point. Note that this does not block #1632, as actions can work with requirements or pyproject.toml files.

[RobPasMue]

Definitely! This is a low-hanging fruit. Let's pair up later @clatapie =)