BF: Migrate CRCNS DataCite fetch to /dois endpoint

[yarikoptic]

DataCite retired the legacy /works REST endpoint (returns 410 Gone with a pointer to /dois, /providers, /clients), which broke test_get_metadata in CI. Switch fetch_datacite_metadata to
/dois?prefix=10.6080&detail=true&page[size]=1000 — prefix 10.6080 is the CRCNS.org DOI namespace, and detail=true is required to include the base64-encoded XML in attributes.xml (list responses omit it by default).

Also swap base64.decodestring for base64.b64decode (the former was removed in Python 3.9) and update the outdated search.datacite.org host check in the skip-on-502 fallback to api.datacite.org so the test still skips gracefully on transient upstream errors.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.94%. Comparing base (bf7c4c6) to head (7ae38c0).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
datalad_crawler/pipelines/tests/test_crcns.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   76.58%   76.94%   +0.35%     
==========================================
  Files          58       58              
  Lines        4733     4733              
  Branches      596      596              
==========================================
+ Hits         3625     3642      +17     
+ Misses        947      928      -19     
- Partials      161      163       +2     

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