Add _python_set_IUSE().
(Patch by Arfrever. Backported from Progress Overlay.)
--- python.eclass
+++ python.eclass
@@ -106,6 +106,28 @@
 	fi
 }
 
+_python_set_IUSE() {
+	local PYTHON_ABI USE_flags
+
+	_PYTHON_LOCALLY_SUPPORTED_ABIS=()
+
+	for PYTHON_ABI in "${_PYTHON_GLOBALLY_SUPPORTED_ABIS[@]}"; do
+		if ! _python_check_python_abi_matching --patterns-list "${PYTHON_ABI}" "${PYTHON_RESTRICTED_ABIS}"; then
+			_PYTHON_LOCALLY_SUPPORTED_ABIS+=("${PYTHON_ABI}")
+			USE_flags+="${USE_flags:+ }python_abis_${PYTHON_ABI}"
+		fi
+	done
+
+	if ! has "${EAPI:-0}" 4; then
+		IUSE="${USE_flags}"
+	fi
+}
+
+if ! has "${EAPI:-0}" 0 1 2 3 && _python_package_supporting_installation_for_multiple_python_abis; then
+	_python_set_IUSE
+fi
+unset -f _python_set_IUSE
+
 # @ECLASS-VARIABLE: PYTHON_DEPEND
 # @DESCRIPTION:
 # Specification of dependency on dev-lang/python.