Use codecov instead of coveralls

[astrofrog]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@6c8bec9). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #17   +/-   ##
=========================================
  Coverage          ?   83.43%           
=========================================
  Files             ?        2           
  Lines             ?      163           
  Branches          ?        0           
=========================================
  Hits              ?      136           
  Misses            ?       27           
  Partials          ?        0

Impacted Files	Coverage Δ
pytest_arraydiff/plugin.py	83.33% <ø> (ø)

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