build: remove unnecessary codecov dependency & upgrade GHA

[matthugs]

Context: https://about.codecov.io/blog/message-regarding-the-pypi-package/

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b8139aa) 100.00% compared to head (32f28e6) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #276   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          436       436           
  Branches        47        47           
=========================================
  Hits           436       436           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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[timmc-edx]

ci.in should only pull in tox and tox-battery -- tox manages its own virtualenv where it pulls in coverage (via test.txt I believe).

[timmc-edx]

(so this line should just be deleted)

[timmc-edx]

Actually, it looks like Feanil beat you to it: #274

[matthugs]

Indeed @feanil did! Thanks @timmc-edx & Feanil!