Revert "python*-r1.eclass: Replace non-portable use of 'declare -g'"

This reverts commit 44d3345d82.

This was triggering "No supported implementation in PYTHON_COMPAT.".

eclass/python-single-r1.eclass

@@ -238,7 +238,7 @@ _python_single_set_globals() {
 			PYTHON_DEPS+="python_single_target_${i}? ( ${PYTHON_PKG_DEP} ) "
 		done
 	fi
-	readonly PYTHON_USEDEP=${optflags// /,}
+	declare -g -r PYTHON_USEDEP=${optflags// /,}
 
 	# 1) well, python-exec would suffice as an RDEP
 	# but no point in making this overcomplex, BDEP doesn't hurt anyone