Fix issue saving S4 objects as RDS

[jkanche]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 91.97531% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.01%. Comparing base (8e014c6) to head (7c32f6b).

Files with missing lines	Patch %	Lines
src/rds2py/save_compressed_list.py	68.18%	6 Missing and 1 partial ⚠️
src/rds2py/read_granges.py	80.00%	0 Missing and 4 partials ⚠️
src/rds2py/read_sce.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #77       +/-   ##
===========================================
+ Coverage   83.54%   95.01%   +11.46%     
===========================================
  Files          29       29               
  Lines         924     1064      +140     
  Branches      137      165       +28     
===========================================
+ Hits          772     1011      +239     
+ Misses         74       20       -54     
+ Partials       78       33       -45     

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