Fix formatting new warning

[prisae]

#384 popped up as warning in my tests, where is saw the missing white spaces ("berepeatable" and "isnot"):

  /home/dtr/Codes/emg3d/tests/test_simulations.py:959: UserWarning: You are running a pytest without setting a random seed, the results might not berepeatable. For repeatable tests please pass an argument to `random seed` that isnot `None`.
    discretize.tests.assert_isadjoint(

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.21%. Comparing base (bc3e78d) to head (68f456f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #390   +/-   ##
=======================================
  Coverage   86.21%   86.21%           
=======================================
  Files          90       90           
  Lines       18625    18625           
  Branches     2103     2103           
=======================================
  Hits        16058    16058           
  Misses       1883     1883           
  Partials      684      684           

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