dev-python/python-box: Make C extensions optional

Signed-off-by: Michał Górny <mgorny@gentoo.org>

dev-python/python-box/metadata.xml

@@ -13,4 +13,7 @@
 		<remote-id type="github">cdgriffith/Box</remote-id>
 		<remote-id type="pypi">python-box</remote-id>
 	</upstream>
+	<use>
+		<flag name="native-extensions">Compiles native C extensions</flag>
+	</use>
 </pkgmetadata>

dev-python/python-box/python-box-7.1.1.ebuild

@@ -24,6 +24,7 @@ S=${WORKDIR}/${MY_P}
 LICENSE="MIT"
 SLOT="0"
 KEYWORDS="~amd64"
+IUSE="+native-extensions"
 
 RDEPEND="
 	dev-python/msgpack[${PYTHON_USEDEP}]
@@ -34,11 +35,23 @@ RDEPEND="
 	dev-python/tomli-w[${PYTHON_USEDEP}]
 "
 BDEPEND="
-	dev-python/cython[${PYTHON_USEDEP}]
+	native-extensions? (
+		dev-python/cython[${PYTHON_USEDEP}]
+	)
 "
 
 distutils_enable_tests pytest
 
+src_prepare() {
+	if ! use native-extensions; then
+		# a cheap hack, extensions are auto-disabled if Cython.Build
+		# is not importable
+		> Cython.py || die
+	fi
+
+	distutils-r1_src_prepare
+}
+
 python_test() {
 	rm -rf box || die
 	epytest