Add pytest CI and pre-commit

[Illviljan]

Add a pytest CI with different OS'.

• When I run the tests on my windows pc the tests fail because of os.linesep.

This adds https://pre-commit.com/ to the project.

I suggest to also add pre-commit to the CI, https://pre-commit.ci/, so new contributors doesn't have to install pre-commit and run it themself.
I think only maintainers can activate the pre-commit.ci though.

Locally the workflow is:

• cd to dcmReader root directory
• Run pre-commit run --all-files --hook-stage manual

The config was based on:

• https://github.com/pydata/xarray/blob/main/.pre-commit-config.yaml
• https://github.com/pydata/xarray/blob/main/setup.cfg

Notable things:

• Clean up trailing whitespace etc.
• Enforce absolute imports with absolufy-imports.
• Use ruff for various stuff
• Use black to autoformat the code. Format also docstring examples with blackdoc.
• Run mypy to check type hints. Requires adding --hook-stage manual as it can be slow. There should be a separate workflow for mypy in the CI.