Replace xopen with lenskit.data.xopen

[mdekstrand]

This adds a new xopen function to lenskit.data, replacing the xopen package and reducing our dependency load.

Closes #1238.

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.44%. Comparing base (245875b) to head (f640005).

Files with missing lines	Patch %	Lines
src/lenskit/data/_xopen.py	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1239      +/-   ##
==========================================
- Coverage   90.51%   90.44%   -0.07%     
==========================================
  Files         263      264       +1     
  Lines       17614    17634      +20     
==========================================
+ Hits        15943    15949       +6     
- Misses       1671     1685      +14     

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