Fix Python ImportError when loading data (itsdangerous serializer)

[Prinxeeee]

This PR fixes an ImportError when loading data with newer versions of
itsdangerous, where TimedJSONWebSignatureSerializer is no longer available.

The code now falls back to URLSafeTimedSerializer, maintaining backward
compatibility with older itsdangerous versions.

Testing:

• Verified that running GeoHealthCheck/GeoHealthCheck/models.py no longer raises
  the TimedJSONWebSignatureSerializer ImportError.
• Further execution fails later due to missing dependencies in the local
  Windows/Python 3.13 environment, which is unrelated to this fix.

Fixes #461

[Prinxeeee]

Hi maintainers,

Just a gentle ping on this PR in case it slipped through the cracks.

Happy to make any changes or add tests if needed. Thanks for taking a look!