Allow future extension of the spec in a backward compatible way (#1578)

pep-0631.rst

@@ -41,12 +41,12 @@ Specification
 dependencies
 ------------
 
-- Format: array of strings
+- Format: array of strings or inline tables
 - Related core metadata:
 
   - `Requires-Dist`_
 
-Every element must be an `entry <#entries>`_.
+Every element SHOULD be an `entry <#entries>`_.
 
 ::
 
@@ -57,6 +57,9 @@ Every element must be an `entry <#entries>`_.
       'subprocess32; python_version < "3.2"',
     ]
 
+Any element MAY be an inline table to support possible future functionality.
+Backends MUST silently ignore these until the specification is extended.
+
 optional-dependencies
 ---------------------
 
@@ -67,7 +70,7 @@ optional-dependencies
   - `Requires-Dist`_
 
 Each key is the name of the provided option, with each value being the same type as
-the `dependencies <#dependencies>`_ field i.e. an array of strings.
+the `dependencies <#dependencies>`_ field i.e. an array of strings or inline tables.
 
 ::