Implement caching for PersonIndex

[mbollmann]

First attempt at implementing caching for the library, targeting PersonIndex.

In my testing, calling anthology.people.load() takes ca. 7 seconds when building from the XML files, and ~1.5 seconds when loading from the cache file.

However, when calling anthology.load_all(), the difference is much less pronounced — maybe it's worth testing how much Collection objects would profit from caching as well.

Finally, I encountered some issues during local testing where occasionally the cache file would be loaded even though it was empty, so despite the existing tests there are some more kinks to be ironed out.

[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.32%. Comparing base (d4c7d10) to head (9b57e12).

Files with missing lines	Patch %	Lines
python/acl_anthology/people/index.py	96.96%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           python-dev    #5782      +/-   ##
==============================================
+ Coverage       95.28%   95.32%   +0.03%     
==============================================
  Files              35       35              
  Lines            3031     3077      +46     
==============================================
+ Hits             2888     2933      +45     
- Misses            143      144       +1     

Files with missing lines	Coverage Δ
python/acl_anthology/anthology.py	89.94% <100.00%> (+0.56%)	⬆️
python/acl_anthology/collections/collection.py	98.41% <ø> (-0.02%)	⬇️
python/acl_anthology/config.py	100.00% <100.00%> (ø)
python/acl_anthology/people/name.py	98.01% <ø> (ø)
python/acl_anthology/people/person.py	100.00% <100.00%> (ø)
python/acl_anthology/people/index.py	97.61% <96.96%> (-0.09%)	⬇️

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[mjpost]

Just request the review when you want it.