fix: codecov not running on PRs

[syedsajjadkazmii]

Description

Moving back to codecov v3. Details: openedx/wg-frontend#177 (comment)

Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Sandbox, if applicable.
• Is there adequate test coverage for your changes?

Post-merge Checklist

• Deploy the changes to prod after verifying on stage or ask @openedx/2u-vanguards to do it.
• 🎉 🙌 Celebrate! Thanks for your contribution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.71%. Comparing base (8be350e) to head (855f5ad).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1213      +/-   ##
==========================================
+ Coverage   84.22%   87.71%   +3.49%     
==========================================
  Files         126      124       -2     
  Lines        2396     2288     -108     
  Branches      680      636      -44     
==========================================
- Hits         2018     2007      -11     
+ Misses        358      272      -86     
+ Partials       20        9      -11     

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