Update PYTHON.json current value for version field in documentation (#518)

Hi,

Just a minor PR to update `PYTHON.json`'s `version` field value in
documentation, following this change


0d7e5e0#diff-e777e81ce98aba56bbd99a66c610b273b2869e8c2c048b39eb75c74c83d3b793R1760

Author: touilleMan

docs/distributions.rst

@@ -38,7 +38,7 @@ without having to resort to heuristics.
 The file contains a JSON map. This map has the following keys:
 
 version
-   Version number of the file format. Currently ``7``.
+   Version number of the file format. Currently ``8``.
 
 target_triple
    A target triple defining the platform and architecture of the machine