Minor tweaks to readme and metadata

[medley56]

• Remove references to old GitHub repo path
• Add PyHC reference
• Update IMAP image link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.82%. Comparing base (fd157b8) to head (b8454fc).
Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #146   +/-   ##
=======================================
  Coverage   91.82%   91.82%           
=======================================
  Files          35       35           
  Lines        2543     2543           
=======================================
  Hits         2335     2335           
  Misses        208      208           

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