BLD: Allow building with NumPy nightlies and use it for nightlies
I am not sure that this is uncontested that this is a good version pin change.  I also took the liberty of removing the current use of oldest-support-numpy, since Python 3.9 means that you should normally always get 1.25 anyway and that should work for any NumPy version.

The `<=2.0.0.dev0` is a way to explicitly allow NumPy nightly wheels, although of course you need the additional URL to use it. In a release version, it might be nicer to not have it, but it also should not really hurt since the you are unlikely to have the NumPy dev wheels available unless you really want them.

(Not sure how to best test that the changes work as is.)

---

@lithomas1 I suppose you are the one to discuss this?  I am not sure that others will agree that changing the version pin as I did is a viable approach, yet.
Maybe there is some reason why removing oldest-support-numpy seems terrible also?!  To me it seems like it should be safe, assuming it works.
Maybe the better path is to just install NumPy explicitly.  cibuildwheel shouldn't use build isolation by default (I think).  So adding an explicit install to the pre-build step should work.  (And no need for changing the version pin.) thanks @seberg.

Let's put this in as is for now. I'll fiddle with it a little more if it ends up breaking something.