Bump min Python version 3.7

[PicoCentauri]

Removes support for Python 3.7 as already merged in metatensor#425.

Additionally I cleaned up the tox.ini file. I moved the environment descriptions from a comment to the description tag. This makes the descriptions visible from the CLI via tox list:

default environments:
lint         -> lint the Python code with flake8 (code linter), black (code formatter), and isort (sorting of imports)
min-deps     -> Run Python unit tests with the minimal dependencies installed
all-deps     -> Run Python unit tests with all dependencies installed (ase & chemfiles are optional dependencies)
docs-tests   -> Runs the doctests defined in any metatensor package
torch-tests  -> Run Python unit tests using torch

additional environments:
docs         -> Build the package documentation.
format       -> Abuse tox to do actual formatting on all files.
build-python -> [no description]

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📚 Documentation preview 📚: https://rascaline--262.org.readthedocs.build/en/262/

[Luthaf]

Thanks a lot!

[github-actions]

Here is a pre-built version of the code in this pull request: wheels.zip, you can install it locally by unzipping wheels.zip and using pip to install the file matching your system