TST Fix test_memmapping for Python nogil

[lesteve]

Another test fix for Python nogil

[codecov]

Codecov Report

Base: 93.89% // Head: 93.89% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (86d88cc) compared to base (e4c00de).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1395   +/-   ##
=======================================
  Coverage   93.89%   93.89%           
=======================================
  Files          52       52           
  Lines        7341     7342    +1     
=======================================
+ Hits         6893     6894    +1     
  Misses        448      448           

Impacted Files	Coverage Δ
joblib/test/test_memmapping.py	99.25% <100.00%> (+<0.01%)	⬆️
joblib/hashing.py	89.47% <0.00%> (-1.76%)	⬇️
joblib/_parallel_backends.py	92.27% <0.00%> (+0.73%)	⬆️

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[tomMoral]

LGTM, thx @lesteve

[tomMoral]

Merging as the thread_bomb_mitigation in pypy already fails on master.