Refactor to poetry groups instead of extras

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• ~Name of contributors Added to AUTHORS.rst~
• ~Description in RELEASE_NOTES.md Added~

Description of PR

This PR does a few things:

• Refactor to use groups in pyproject.toml instead of extras
• Only import wbdata and unfccc when needed, skip tests if not installed This as a reaction to https://github.com/OliverSherouse/wbdata/issues/74

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 95.1%. Comparing base (ddbb88e) to head (cf75d2e). Report is 2 commits behind head on main.

:exclamation: Current head cf75d2e differs from pull request most recent head 4ca5641. Consider uploading reports for the commit 4ca5641 to get more accurate results

Files	Patch %	Lines
tests/test_data_worldbank.py	50.0%	2 Missing :warning:
pyam/unfccc.py	0.0%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #833 +/- ## ======================================= + Coverage 95.0% 95.1% +0.1% ======================================= Files 64 63 -1 Lines 6134 6142 +8 ======================================= + Hits 5828 5842 +14 + Misses 306 300 -6 ```

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[danielhuppmann]

Please review @glatterf42