mesonlib: use collections.abc for abc types

The use of ABC classes (like MutableSet) is deprecated currently, in
python 3.8 the aliases in collections will be dropped and only the ones
in collections.abc will remain. collections.abc has existed since python
3.3, so there is no backwards compatibility risk.

mesonbuild/mesonlib.py

@@ -1062,7 +1062,7 @@ def substring_is_in_list(substr, strlist):
             return True
     return False
 
-class OrderedSet(collections.MutableSet):
+class OrderedSet(collections.abc.MutableSet):
     """A set that preserves the order in which items are added, by first
     insertion.
     """