Improve GzipReader compression rates.

[rhpvorderman]

Checklist

• Pull request details were added to CHANGELOG.rst
• Documentation was updated (if needed)

[codecov]

Codecov Report

Merging #106 (589824a) into develop (21f1f94) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #106      +/-   ##
===========================================
- Coverage    95.21%   95.18%   -0.04%     
===========================================
  Files            2        2              
  Lines          251      249       -2     
  Branches        65       65              
===========================================
- Hits           239      237       -2     
  Misses           5        5              
  Partials         7        7              

Impacted Files	Coverage Δ
src/isal/igzip.py	95.04% <100.00%> (-0.05%)	⬇️

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[rhpvorderman]

Fixes #71