Support for Slurm 22.05

[tazend]

Adds support for Slurm 22.05

• delete obsolote jinja2 directory
• restructure how C-API code is organized -> seperate slurm/ directory -> slurm.pxd single file deleted
• xmalloc.h deleted -> very old definitions, we can wrap around xcalloc from libslurm.so instead
• adds a script pyslurm_bindgen.py which automatically generates new header definitions from the c headers (when doing major upgrades, like this one)
  • also add a small guide how it's done

Few folks have already tested the api against slurm-web, see #237 Have already created a 21.08 branch from the current master (to have these major releases separated)

May have to check whether the pytests all pass and to make the CI happy

[tazend]

@giovtorres after this is merged and before making a release ready, I'd go ahead and make another PR which solves #190 and #236 , which I have pretty much done (and can do PR today/tomorrow), so the release can also directly benefit especially from changes which come from #190 to enhance the build-system (e.g. pyproject.toml)