Fix python 3.12 testbank

[veenstrajelmer]

Python 3.12 is available since 2023-10-02 (https://devguide.python.org/versions). A testbank was added, but there are several issues with setting up the environment.

Issues:

• py3.12 was not available, but it is GA since 2 October 2023
• fiona installation failed: Build and upload Python 3.12 wheels sgillies/fiona-wheels#38
• netcdf installation failed, but now works
• numba installation fails: Python 3.12 numba/numba#9197 >> expected couple of weeks after 27th October 2023
• copernicus-marine-client requires "cachier>=2.2.1" requires pathtools requires imp which fails to build: Does not build on python 3.12 python-cachier/cachier#129 >> pathtools dependency was removed
• copernicus-marine-client requires "aiohttp>=3.8.5" requires frozenlist and multidict which fails to build: (🎁) Build wheels for 3.12 aio-libs/frozenlist#541 and Build wheels for Python 3.12 aio-libs/multidict#887
• There might be more packages that have this issue: https://pyreadiness.org/3.12/

If py3.12 testbank is green:

• add to general pytest workflow file (runs on every PR and merge)
• remove dedicated py312 workflow file
• add py3.12 to supported versions in pyproject.toml
• updated readme/installation/contributing docs