Add retry mechanism for HTTP requests and update dependencies

[mjanez]

This pull request includes several changes to improve the robustness and reliability of HTTP requests in the ckan2pycsw script and updates to the project dependencies. Key changes include the addition of a retry mechanism for HTTP requests, improvements to error handling, and updates to the pyproject.toml file.

Improvements to HTTP requests and error handling:

• ckan2pycsw/ckan2pycsw.py: Added Retry and HTTPAdapter from the requests library to configure retries for HTTP requests, and updated the get_datasets function to use this session with retries and a timeout. Improved error handling by adding specific exceptions for Timeout and ConnectionError. [1] [2] [3]

Dependency updates:

• pyproject.toml: Added urllib3 to the dependencies to support the retry mechanism.
• pyproject.toml: Removed an unnecessary section from the tool.pdm configuration.