remove nep18 compatibility code

[keewis]

Since we dropped support for numpy<1.17 we can stop checking whether NEP18 has been activated.

• Passes pre-commit run --all-files

[mathause]

LGTM. Can we get rid of the ubuntu-latest py37-min-nep18 CI?

[keewis]

not sure. I think it was added because as per the policy we only support the latest version of NEP18 packages. These required numpy>=1.16, but now that we require numpy>=1.17 that's not that much of an issue. Also, we don't have the min-deps CI check that environment so I seem to have missed that when bumping the versions (it still installs pandas=0.25, pint=0.15, and sparse=0.8). It would probably be good to have a separate issue / PR to discuss these things.

Related to that, I just noticed that the "Installation" section in our docs is really out-of-date (among others it lists numpy=1.15 as minimum requirement and the recommends to install pint from a dev branch). I'll send in a PR to fix those.

[dcherian]

Thanks @keewis