zopen: add explicit encoding to read_json; explicit mode

[rkingsbury]

Add explicit mode and encoding arguments to all zopen calls, to avoid warnings associated with recent monty changes.

See materialsvirtuallab/monty#730

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.91%. Comparing base (0fddbc2) to head (02520e8).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1030   +/-   ##
=======================================
  Coverage   88.91%   88.91%           
=======================================
  Files          46       46           
  Lines        3997     3997           
=======================================
  Hits         3554     3554           
  Misses        443      443           

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