Fix skipping entrypoints test

[dstansby]

Fixes https://github.com/zarr-developers/numcodecs/issues/596

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.91%. Comparing base (e3299b8) to head (f8bbe1b). Report is 59 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #618   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          59       59           
  Lines        2405     2406    +1     
=======================================
+ Hits         2403     2404    +1     
  Misses          2        2           

Files with missing lines	Coverage Δ
numcodecs/tests/test_entrypoints_backport.py	100.00% <100.00%> (ø)