ci: switch to coveralls

[onerandomusername]

Relevant Issues

closes #112

Description

remove codecov and switch to coveralls

[codecov]

Codecov Report

Merging #127 (d0d7d78) into main (42e8674) will not change coverage.
The diff coverage is n/a.

❗ Current head d0d7d78 differs from pull request most recent head a0fb30a. Consider uploading reports for the commit a0fb30a to get more accurate results

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage   70.90%   70.90%           
=======================================
  Files          33       33           
  Lines        1619     1619           
  Branches      199      199           
=======================================
  Hits         1148     1148           
  Misses        439      439           
  Partials       32       32           

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[coveralls]

Pull Request Test Coverage Report for Build 1509571551

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 20 unchanged lines in 4 files lost coverage.
• Overall coverage remained the same at ?%

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Files with Coverage Reduction	New Missed Lines	%
modmail/config.py	1	97.18%
tests/modmail/extensions/test_plugin_manager.py	4	81.82%
modmail/plugin_helpers.py	6	0%
modmail/utils/plugins.py	9	54.9%

Totals
Change from base Build #1:	0%
Covered Lines:
Relevant Lines:	0

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💛 - Coveralls