Set character encoding explicitly in JSONStore

[rkingsbury]

Fixes #122

Prior to this PR, since the character encoding was not explicitly set, trying to read the database files on non-English platforms (or any platforms where the encoding is not UTF 8) would result in a UnicodeDecodeError. This PR fixes the problem by implementing the new encoding kwarg recently added to maggma.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.52%. Comparing base (a2ebba3) to head (3ae9a6e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #134   +/-   ##
=======================================
  Coverage   81.52%   81.52%           
=======================================
  Files           9        9           
  Lines        1429     1429           
  Branches      294      294           
=======================================
  Hits         1165     1165           
  Misses        224      224           
  Partials       40       40           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.