Remove dead code and stale Python 2 leftovers (with the help of Claude Fable)

[Carreau]

• ClassTypes tuple in traitlets.py, a Python 2/3 compat constant never used and not exported
• execfile() shim in config/loader.py, shadowing the removed Python 2 builtin, never called
• Config._merge in config/loader.py, a deprecated private alias for Config.merge that was never called and never warned

[ccordoba12]

Config._merge in config/loader.py, a deprecated private alias for Config.merge that was never called and never warned

@Carreau, unfortunately this change broke both Spyder and Spyder-kernels. Could you release 5.16.1 reverting it? That would give us time to address it in versions 6.1.7/3.1.6 of those packages, to be released at the end of August.

By the way, I don't remember why we're using _merge (perhaps because there was no merge at the time we wanted to merge configs), but it's been a stable API for at least 10 years.

[Carreau]

just _merge? nothing else ?

I'll do that on monday.

[Carreau]

Please feel free to send a PR next time (And see #963 for _merge)

And _merge had been marked as deprecated for at least 11 years; plus there is a leading underscore, so it's technically private.

[ccordoba12]

just _merge? nothing else ?

I think so. I mean, that's what I saw in our failing tests. But I'll check more thoroughly during the weekend.

I'll do that on monday.

Thanks!

Please feel free to send a PR next time (And see #963 for _merge)

Will do. I just wanted to see first if you were ok with the idea.

And _merge had been marked as deprecated for at least 11 years; plus there is a leading underscore, so it's technically private.

As I said, I don't recall why we use _merge. Perhaps we started to use it more than 11 years ago. And sure, we should have changed it but you know how the life of a maintainer is.

[Carreau]

We should also start to publish nightly of traitlets maybe.