Update Codecov upload API to v4

[ararslan]

Guessing based on https://docs.codecov.io/reference#upload.

[ararslan]

RIP, looks like there's no CI here anymore. Guess we'll need to set up GHA.

[vtjnash]

Can you rebase and re-enable CI submission on https://github.com/JuliaCI/Coverage.jl/blob/master/etc/travis-coverage.jl#L4

[ararslan]

I've rebased. I don't know what you mean about reenabling submission but feel free to push to this branch.

[vtjnash]

You'll need to remove the conditional on that line. Looks like you might need to disable the blackhole server that we use for testing also (or provide a way to specify the API via an environment variable, and use v2 there?)

[DilumAluthge]

I have rebased, and I have enabled submission (i.e. we've removed all conditionals from https://github.com/JuliaCI/Coverage.jl/blob/master/etc/travis-coverage.jl)

However, it looks like CI is failing on the PR.

(For reference, CI is passing on master.)

[vtjnash]

Yeah, it fails on the blackhole server we used for testing.

[DilumAluthge]

@ararslan @vtjnash Based on #316, it seems like the only issue with this PR is the blackhole server bug. How do y'all want to proceed?

[ararslan]

¯\_(ツ)_/¯

[DilumAluthge]

@vtjnash Can you explain the purpose of the black-hole server?

[vtjnash]

It avoided cluttering the list of log files with duplicate entries–no practical benefit to it

[DilumAluthge]

Alright, I think I have a way to fix it. Basically:

1. If we are not using the "black hole" server, we upload to S3
2. If we are using the "black hole" server, we do not upload to S3

See PR #318.

For the diff between this PR and PR #318, see here: aa/codecov-v4...dpa/codecov-v4