Codecov version 4

[williamjameshandley]

Description

Following the anesthetic PR handley-lab/anesthetic#377
, this PR upgrading the codecov action from v1 to v4, and supplying a CODECOV_TOKEN secret.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4763205) to head (55a5afc).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #24   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines          843       844    +1     
=========================================
+ Hits           843       844    +1     

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[AdamOrmondroyd]

lgtm