Fix Python 3.12 compatibility

[mortenwh]

Replace deprecated pkg_resources.resource_string with importlib.resources.files().joinpath().read_bytes() across all source and test files. pkg_resources is no longer available without an explicit setuptools dependency in Python 3.12+.

Add missing responsible_unit field to storage_information in mmd_elements.yaml to match the MMD XSD schema.

Fix test isolation issue in test_nc_to_mmd.py by closing netCDF4 Dataset handles after use in test_separate_repeated and testNc_to_mmd_get_geographic_extent_polygon.

Update pyproject.toml:

• Replace Python 3.8 classifier with 3.12
• Set requires-python >= 3.9 (importlib.resources.files requires Python 3.9+)
• Relax netCDF4 constraint from ~=1.5 to >=1.5

Fixes #361

Summary: closes #361

Related issue: #361

Suggested reviewer(s):

Reviewer checklist:

• The headers of all files contain a reference to the repository license (i.e., "License: This file is part of py-mmd-tools, licensed under the Apache License 2.0 (https://www.apache.org/licenses/LICENSE-2.0)")
• 100% test coverage of new code - meaning:
  • The overall test coverage increased or remained the same as before
  • Every function is accompanied with a test suite
  • Tests are both positive (testing that the function work as intended with valid data) and negative (testing that the function behaves as expected with invalid data, e.g., that correct exceptions are thrown)
  • Functions with optional arguments have separate tests for all options
• Examples are supported by doctests
• All tests are passing
• All names (e.g., files, classes, functions, variables) are explicit
• Documentation (as docstrings) is complete and understandable

The checklist is based on the S-ENDA conventions and definition of done (see https://s-enda-documentation.readthedocs.io/en/latest/general_conventions.html). The above points are not necessarily relevant to all contributions. In that case, please add a short explanation to help the reviewer.