Pin unfccc-di-api to latest release and reactivate tests

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR reverts #646 after @mikapfl fixed primap-community/unfccc_di_api#37.

[codecov]

Codecov Report

Merging #647 (5edf2fe) into main (e33fe00) will increase coverage by 0.6%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #647     +/-   ##
=======================================
+ Coverage   93.9%   94.5%   +0.6%     
=======================================
  Files         57      57             
  Lines       5662    5662             
=======================================
+ Hits        5317    5356     +39     
+ Misses       345     306     -39     

Impacted Files	Coverage Δ
tests/test_unfccc.py	100.0% <100.0%> (+53.8%)	⬆️
pyam/unfccc.py	89.7% <0.0%> (+65.3%)	⬆️

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