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-r1.eclass

@@ -216,8 +216,8 @@ _python_set_globals() {
 	optflags+=,${flags_st[@]/%/(-)}
 
 	IUSE=${flags[*]}
-	readonly PYTHON_REQUIRED_USE="|| ( ${flags[*]} )"
-	readonly PYTHON_USEDEP=${optflags// /,}
+	declare -g -r PYTHON_REQUIRED_USE="|| ( ${flags[*]} )"
+	declare -g -r PYTHON_USEDEP=${optflags// /,}
 
 	# 1) well, python-exec would suffice as an RDEP
 	# but no point in making this overcomplex, BDEP doesn't hurt anyone