expose encoding with default utf-8 for reading xml files

[gboeing]

Resolves #1083

It seems that some Windows installations do not open XML files with utf-8 encoding even when 1) the file is encoded as utf-8, 2) the XML declaration states it is utf-8, and 3) the sys.getfilesystemencoding() is utf-8.

This seems to be because the system's locale.getpreferredencoding() sometimes defaults to cp1252 instead. This PR exposes an encoding parameter to the graph_from_xml and features_from_xml functions so a user can manually specify a certain encoding if they need to.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.83%. Comparing base (ef5d465) to head (ec41824).
Report is 676 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1084   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files          27       27           
  Lines        2497     2497           
=======================================
  Hits         2443     2443           
  Misses         54       54           

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