pyproject.toml: move build-system bits where they belong

[akx]

These had been misplaced in 5d3f9fd.

Moving them back to the [build-system] section makes this package compliant with PEP 517, and e.g. installing from a git+ URL (or a working copy) works once more:

85baacc

$ pip install /repo
Processing /repo
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Building wheels for collected packages: UNKNOWN
  Building wheel for UNKNOWN (PEP 517) ... done
  Created wheel for UNKNOWN: filename=UNKNOWN-0.0.0-py3-none-any.whl size=1803 sha256=20f675a1a4ffc1e82b9102e96139701b0b73bfd2f1c21f5bff73a5f98473e86e
Successfully built UNKNOWN
Installing collected packages: UNKNOWN

this PR

# pip install /repo
Processing /repo
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Collecting dependencies
Building wheels for collected packages: django-q
  Building wheel for django-q (PEP 517) ... done
  Created wheel for django-q: filename=django_q-1.3.9-py3-none-any.whl size=89882 sha256=133626bc462890c671cca41183a4581b027745230714dff79a7eef57f836a5ba
Successfully built django-q
Installing collected packages: sqlparse, six, asgiref, wcwidth, python-dateutil, django, redis, django-picklefield, blessed, arrow, django-q