ENH: delay expensive imports

[neutrinoceros]

PR Summary

Reduces yt's startup time by ~30% (from 1.57s to 1.07s on my machine, averaged over 100 runs)

Measured with this simple script

from time import monotonic_ns

tstart = monotonic_ns()
import yt
tstop = monotonic_ns()
print(f"{(tstop-tstart)/1e9:.3f}")                                                                      

[neutrinoceros]

Getting some errors at collection time in minimal env tests. I don't immediately get what's happening so I'll switch to draft for now.

[neutrinoceros]

I found that urllib.request, which is probably never actually used within yt, costs about 1% (and 2.5% with incoming improvements in our other dependencies) of startup overhead. It's definitely in the realm of fine tuning but it's basically free to implement, so I went for it too.

[matthewturk]

this is a good thing to know about!