Modernize CI, add dev dependencies

[sbiquard]

• Drop unused CONDA_ENV/CACHE_NUMBER vars
• Drop Fortran toolchain setup: camb ships prebuilt wheels, and pymaster does not need Fortran
• Drop manual pip install numpy/scipy/healpy/...: these are already declared as soopercool dependencies in pyproject.toml
• Drop unused coveralls install (never invoked)
• Move flake8/pytest/pytest-cov into a dev dependency group in pyproject.toml instead of installing them ad hoc in the workflow
• Bump actions/checkout to a recent version
• Install everything via uv on Python 3.13
• Delete .github/environment.yml: unreferenced by the workflow, likely dead
• uv venv puts .venv inside the repo, so add it to flake8's exclude list in .flake8
• Drop dead global declarations in mpi_utils.py: no assignments in that scope, so global does nothing there (newly flagged by pyflakes' F824 check added in pyflakes 3.3.0, which wasn't around under the old CI's pinned flake8<7.2 / pyflakes<3.3.0).

[adrien-laposta]

This sounds good, happy to merge