Backport PR #874 on branch 1.x (Fix gateway kernel shutdown)

[meeseeksmachine]

Backport PR #874: Fix gateway kernel shutdown

[codecov-commenter]

Codecov Report

Merging #875 (36ce714) into 1.x (7bbc200) will increase coverage by 0.09%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##              1.x     #875      +/-   ##
==========================================
+ Coverage   70.33%   70.42%   +0.09%     
==========================================
  Files          62       62              
  Lines        7652     7655       +3     
  Branches     1269     1270       +1     
==========================================
+ Hits         5382     5391       +9     
+ Misses       1886     1880       -6     
  Partials      384      384              

Impacted Files	Coverage Δ
jupyter_server/gateway/managers.py	60.10% <61.53%> (+1.87%)	⬆️

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[kevin-bates]

Thank you @blink1073!