fix: Use urllib to make _offline() aware of HTTP(s) proxies.

[MeraX]

Description

The socket package does not respect the environment variable http_proxy . However most (all) download methods used in utils consider proxies. Therefore, the _offline test should work with proxies as well.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Issue Number

Enable tests on systems that require HTTP proxies.

Code Compatibility

• I have performed a self-review of my code

Code Performance and Testing

• I have added tests that prove my fix is effective or that my feature works
• I ran the complete Pytest test suite locally, and they pass

Dependencies

• I have ensured that the code is still pip-installable after the changes and runs
• I have tested that new dependencies themselves are pip-installable.

Documentation

• My code follows the style guidelines of this project
• I have updated the documentation and docstrings to reflect the changes
• I have added comments to my code, particularly in hard-to-understand areas

Additional Notes

[FussyDuck]

All committers have signed the CLA.