python.mk: use gpep517 as build frontend on Python 3.7

Should start allowing PEP-517 packages to build and unbreaking
circular dependencies, however short a shelf life, due to gpep517's
almost nonexistant Python dependency tree.

(PyPA build will continue as the preferred build frontend otherwise,
due to official stewardship, but does not preclude a DEFAULT_VERSIONS
hook in the future)