[issue39777] Use the codecov GH Action

[Brett Cannon]

New submission from Brett Cannon <brett at python.org>:

Codecov provides a GH Action for uploading coverage reports which we might as well use instead of their bash uploader: https://github.com/marketplace/actions/codecov.

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messages: 362839
nosy: brett.cannon, steve.dower
priority: normal
severity: normal
status: open
title: Use the codecov GH Action

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39777>
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