Fix GitHub actions (Codecov + macOS)

[leifgehrmann]

This issue is similar to: leifgehrmann/pangocffi#57

The GitHub actions are failing again. This is due to two issues:

• v3 of codecov-actions is no longer supported. It's an almost seamless upgrade to v4, but it now requires a token called CODECOV_TOKEN, which I've added to this repository.
• In recent versions of macOS (I suspect macs with M-series chips), homebrew places libraries like pango and cairo in unexpected locations that cause the tests for pangocairocffi to fail to load the library. Users of cairocffi faced similar issues (which I've linked in the yaml) and was solved by creating a symbolic link between the libraries and the pangocairocffi directory. Unsure if this is the right thing to do, but it works, and this is only done for the GitHub action, it's not used anywhere else critical. For reference, the error below is what happens if the library cannot be located.

 >       raise OSError("dlopen() failed to load a library: %s" % ' / '.join(names))
  E       OSError: dlopen() failed to load a library: cairo / cairo-2 / cairo-gobject-2 / cairo.so.2

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.48%. Comparing base (fa59166) to head (d254a6b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   75.48%   75.48%           
=======================================
  Files           6        6           
  Lines         155      155           
=======================================
  Hits          117      117           
  Misses         38       38           

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