Refactor deprecated numpy.core.defchararray methods

[PreisTo]

The defchararray is not recommended since numpy 1.4 and the preferred alias for defchararray is numpy.char.

Refactored the only two occurrences in fermipy - local pytests succeeding without issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.16%. Comparing base (63b8808) to head (685a83c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
- Coverage   51.23%   51.16%   -0.07%     
==========================================
  Files         145      145              
  Lines       27437    27436       -1     
==========================================
- Hits        14056    14037      -19     
- Misses      13381    13399      +18     

Flag	Coverage Δ
unittests	51.16% <100.00%> (-0.07%)	⬇️

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[ndilalla]

Thanks @PreisTo !