package/pkg-python: add setup.cfg-only setuptools project support

Some projects have been removing their setup.py shims under the
assumption that they are not needed due to setuptools being usable
via pep517, however this is not actually the case as it is not
possible to bootstrap setuptools without setup.py style builds as
setuptools has a dependency on wheel when being used via pep517
which is itself not installable via pep517 as that would create a
dependency cycle between wheel and setuptools(which means wheel
itself must use a legacy setup.py build/installation for itself).

In addition our pep517 toolchain itself requires setuptools for
building/installing dependencies, although it would appear these
dependencies will eventually move to flit. Until this is resolved
we must use setup.py style builds for setuptools packages.

Since it is not yet possible to convert setup.py based setuptools
builds to pep517 based setuptools builds we simply need to execute
the setuptools shim code directly if the setup.py file is missing.

See:
https://setuptools.pypa.io/en/latest/setuptools.html#setup-cfg-only-projects

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/pkg-python.mk

@@ -86,6 +86,9 @@ PKG_PYTHON_SETUPTOOLS_ENV = \
 	$(PKG_PYTHON_ENV) \
 	SETUPTOOLS_USE_DISTUTILS=stdlib
 
+PKG_PYTHON_SETUPTOOLS_CMD = \
+	$(if $(wildcard $($(PKG)_BUILDDIR)/setup.py),setup.py,-c 'from setuptools import setup;setup()')
+
 PKG_PYTHON_SETUPTOOLS_INSTALL_OPTS = \
 	--install-headers=/usr/include/python$(PYTHON3_VERSION_MAJOR) \
 	--prefix=/usr \
@@ -180,13 +183,13 @@ endif
 else ifeq ($$($(2)_SETUP_TYPE),setuptools)
 ifeq ($(4),target)
 $(2)_BASE_ENV = $$(PKG_PYTHON_SETUPTOOLS_ENV)
-$(2)_BASE_BUILD_CMD = setup.py build
-$(2)_BASE_INSTALL_TARGET_CMD = setup.py install --no-compile $$(PKG_PYTHON_SETUPTOOLS_INSTALL_TARGET_OPTS)
-$(2)_BASE_INSTALL_STAGING_CMD = setup.py install $$(PKG_PYTHON_SETUPTOOLS_INSTALL_STAGING_OPTS)
+$(2)_BASE_BUILD_CMD = $$(PKG_PYTHON_SETUPTOOLS_CMD) build
+$(2)_BASE_INSTALL_TARGET_CMD = $$(PKG_PYTHON_SETUPTOOLS_CMD) install --no-compile $$(PKG_PYTHON_SETUPTOOLS_INSTALL_TARGET_OPTS)
+$(2)_BASE_INSTALL_STAGING_CMD = $$(PKG_PYTHON_SETUPTOOLS_CMD) install $$(PKG_PYTHON_SETUPTOOLS_INSTALL_STAGING_OPTS)
 else
 $(2)_BASE_ENV = $$(HOST_PKG_PYTHON_SETUPTOOLS_ENV)
-$(2)_BASE_BUILD_CMD = setup.py build
-$(2)_BASE_INSTALL_CMD = setup.py install $$(HOST_PKG_PYTHON_SETUPTOOLS_INSTALL_OPTS)
+$(2)_BASE_BUILD_CMD = $$(PKG_PYTHON_SETUPTOOLS_CMD) build
+$(2)_BASE_INSTALL_CMD = $$(PKG_PYTHON_SETUPTOOLS_CMD) install $$(HOST_PKG_PYTHON_SETUPTOOLS_INSTALL_OPTS)
 endif
 else ifneq ($$(filter flit pep517,$$($(2)_SETUP_TYPE)),)
 ifeq ($(4),target)