Remove outdated self-cimports in pyx files and other Cython 3 prep

[mkoeppe]

These self-imports from the pxd file are no longer needed (even in Cython 0.29.x).

Also replacing Python-2 style relative cimports with absolute cimports throughout.

[tkralphs]

Hi @mkoeppe, I've been watching this PR, but the build is still failing in a way I can't interpret. Are you looking at this? Should I just wait?

[mkoeppe]

Which failures are you referring to? In the checks, the wheel build for PyPy 3.9 is failing, but I don't think it has anything to do with this PR.

[mkoeppe]

In sagemath/sage#36439 I have the final missing piece for switching to Cython 3, but I've backed it out from this PR here to keep the scope limited.

[tkralphs]

Which failures are you referring to? In the checks, the wheel build for PyPy 3.9 is failing, but I don't think it has anything to do with this PR.

OK, I didn't realize that it was the PyPy build that was failing, but yes, that's what I was referring to. Since that build looks to have been working up until this PR, I guessed it must be related. Perhaps that's not the case, but then should we should we either try to figure it out or remove that build before merging?

[mkoeppe]

Let's take that to:

• .github/workflows/cibuildwheel.yml: Update #180

[tkralphs]

Looking good now. Ready to merge?

[mkoeppe]

Yes, please go ahead

[mkoeppe]

As the next step, I will prepare a PR that switches from Cython < 3 to Cython >= 3; that's now #183