Support for 20.11

[tazend]

Hi,

this pull request creates the baseline to make pyslurm compile for 20.11 (Specifically, I compiled with 20.11.8, but as usual any 20.11.* patch should work)

• slurm.pxd now should be again identical to the slurm headers of 20.11 for slurm.h, slurmdbd.h and slurm_errno.h

Also credits to @basvandervlies for modifying the pyslurm.pyx file (provided in #180) adding some of the necessary patches. I included the commit in this PR.

I put some modifications in though:

• powercap stuff seems to be outdated and can be removed, the functions don't exist anymore in the slurm headers

@giovtorres Let me know what you think and if you are also able to compile it.

I also think for the current master branch with 20.2 support a dedicated 20.2 should be created to have that seperated. (basically in the same way the slurm devs do it on github)

closes #189

[giovtorres]

@tazend This looks great! 🎉 And it compiles just fine inside a test Slurm container! Just one small ask to update the version in __version__.py so that it matches the version of Slurm it was compiled against. Then we can ship!

Also, it would be a big help if you could document the steps you took to generate this version. I believe there was a BUILD.md that didn't make it into the main branch. (This doc can be in a follow-up PR.)

[tazend]

@giovtorres Hey, sure I can provide that, makes sense also.

Might probably be worth to put this explanation in a seperate README.md inside the jinja2 directory, to explain the purpose of it, how to "build" the slurm.pxd from the templates and how to go about performing major upgrades pulling in the new header definitions (e.g. using autopxd)

I'll do that in a seperate PR in the next few days.

[giovtorres]

LGTM 🚀 Thank you so much for this contribution!!