Fix memory leak in GzipReader.readall

[rhpvorderman]

Checklist

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

Fixes #166

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7fffa65) 95.12% compared to head (817d418) 95.56%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #167      +/-   ##
===========================================
+ Coverage    95.12%   95.56%   +0.44%     
===========================================
  Files            3        3              
  Lines          451      451              
  Branches        74       74              
===========================================
+ Hits           429      431       +2     
+ Misses          16       15       -1     
+ Partials         6        5       -1     

see 1 file with indirect coverage changes

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