feat: skip upload for sonatype nexus collision

[jafournier]

Context:
We want to use the --skip-existing option with Sonatype Nexus' pypi.
However when package already exists, Sonatype Nexus is delivering a 400 reponse with a content not handled yet in the code.

The purpose of this PR is to handle such contents.

[woodruffw]

Thanks for the PR @jafournier.

Two quick questions:

• Is this different from Sonatype's "Nexus Repository OSS" service? We already support that a few lines above; if this is the new version of that existing service then the old one should probably be removed and replaced with this.
• Is this error message documented anywhere? I couldn't find it with a quick search on the linked docs. This isn't a hard blocker, but it'd be nice to know if this is actually documented vs. something the Sonatype developers might choose to change.

[jafournier]

Thanks for the PR @jafournier.

Two quick questions:

* Is this different from Sonatype's "Nexus Repository OSS" service? We already support that a few lines above; if this is the new version of that existing service then the old one should probably be removed and replaced with this.

* Is this error message documented anywhere? I couldn't find it with a quick search on the linked docs. This isn't a hard blocker, but it'd be nice to know if this is actually documented vs. something the Sonatype developers might choose to change.

Thanks for your answer @woodruffw! Indeed the two are the same, consequently I merged the two lines.
We had a change in behavior when our nexus got bumped to 3.70.3.
I think old-nexus users would like twine to keep the compatibility with their old-nexus.
Unfortunately I haven't found any documentation of such content either.

[woodruffw]

Got it, thanks for checking! This LGTM overall; could you please create a changelog entry for your changes as well? I think this could go under either feature or bugfix (no strong opinion about which).

Instructions for the changelog are here: https://twine.readthedocs.io/en/stable/contributing.html#changelog-entries

[jafournier]

sure, done.

[sigmavirus24]

Yeah so someone was going to fix this in Nexus itself. There is at least one closed issue about this.

[sigmavirus24]

Nexus was supposed to fix this breakage and there's a closed twine issue about that exact topic

[jafournier]

Hello @sigmavirus24, are you refering to #1044 ?
If so this issue is still open, and hasn't been resolved since it's opening, one year ago.

Though I theoretically agree that nexus should homogenize it's error messages and stop changing them, what are the chances the issue would be resolved this coming year in practice?
I you consider the chances are dim, couldn't you accept this tiny modification to enable twine users to use --skip-existing smoothly ?

[sigmavirus24]

@jafournier fixing things here is the path to increasing curb and complexity. Frankly the right fix here is limiting the skip existing feature to indices that actually fully implement the API(s) we expect as we did with trusted publishing and removing string checking hacks.