PEP 735 compliance: dependency groups

[DimitriPapadopoulos]

Only for development and build time dependencies, not run time depdendencies that are meant to be published with the package.

[CPBridge]

Hi @DimitriPapadopoulos

Thanks for your help keeping highdicom up to date with best practices. And sorry for delay from me

I wasn't aware of this PEP until you created this PR. But after quickly reading around it, it seems like a few more tweaks would be necessary to manage knock-on effects of this change on the repo:

• The pip install command here in the github workflow specification would need to be updated to use the --group syntax so that the correct test packages are installed during the testing (as you can see, currently tests are failing due to flake8 being missing)
• The developer documentation describing how to install packages for running tests and building docs would need to be updated to use the --group syntax also.

Are there any other knock-on effects that you can think of?

[DimitriPapadopoulos]

Totally forgot to grep pip install -e .[ and pip install .[ as I usually do. Sorry about it. Will fix this.