Add codecov exclude_lines configuration

[ringohoffman]

Describe your changes

Configure exclude_lines for Codecov. There are certain lines that shouldn't affect code velocity of a PR.

See: nedbat/coveragepy#831 (comment)

Apparently this isn't configurable through codecov.yml 🙈 : codecov/codecov-python#107 (comment)

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

I opened this because I noticed this issue was affecting an existing PR: #1818 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (54e68e0) 55.23% compared to head (48d9b0e) 64.36%.
Report is 68 commits behind head on master.

❗ Current head 48d9b0e differs from pull request most recent head d80f190. Consider uploading reports for the commit d80f190 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1992      +/-   ##
==========================================
+ Coverage   55.23%   64.36%   +9.12%     
==========================================
  Files         300      313      +13     
  Lines       22381    23108     +727     
  Branches     3359        0    -3359     
==========================================
+ Hits        12363    14873    +2510     
+ Misses       9856     8235    -1621     
+ Partials      162        0     -162     

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