Update build-system to allow for editable installs

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 ```