DEPR: deprecate calling yt.load with first argument passed as keyword

[neutrinoceros]

PR Summary

This is a proposed solution to close #2879:
IMO, the name of the first argument to yt.load is bound to be meaningless.
It's current name fn is short for "filename", but we support loading from directories and in some cases, urls !
I think that making it a positional-only argument lifts out the weigh of carying a meaningless name forever.
Now, the difficult part is in continuously evolving the API to avoid sudden breaking changes, which is why 99% of this patch consists in adding a custom decorator to do just that, so existing code that might rely on loading data as yt.load(fn=..., ...) will not immediately break but will still emit some warnings, and we can keep backward compatibility for a while.

TODO:

• add a test for the warning part

[neutrinoceros]

I'll note that the fact that this passed all tests without a warning indicates that calling yt.load(fn=...) is indeed something we never encouraged

[matthewturk]

OK, so, I don't mind this, but I'm surprised it's so much code, and that it's that high of a priority. Do we really need this?

[neutrinoceros]

Not a high priority, just something that was on my mind for 2 years, but required dropping Python 3.7
I don't see another way to close #2879 (other than marking it as 'wontfix').

[matthewturk]

Let's do it