Update build-system to allow for editable installs

[akx]

Before:

$ pip install -e .
Obtaining file:///Users/akx/build/openapi2jsonschema
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
ERROR: Project file:///Users/akx/build/openapi2jsonschema has a 'pyproject.toml' and its build backend is missing the 'build_editable' hook. Since it does not have a 'setup.py' nor a 'setup.cfg', it cannot be installed in editable mode. Consider using a build backend that supports PEP 660.

After:

$ pip install -e .
Obtaining file:///Users/akx/build/openapi2jsonschema
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... done
  Preparing editable metadata (pyproject.toml) ... done
Successfully installed click-7.1.2 colorama-0.4.6 jsonref-0.2 openapi2jsonschema-0.9.0 pyyaml-5.4.1

[fabpiaf]

works, thanks