Issue 39579: Attribute node in a decorator has wrong end_col_offset

There is a problem with the end_col_offset of nested Attribute nodes in decorators. For example, parsing

@a.b.c
def f(): pass

produces the following AST tree (part):

decorator_list=[
    Attribute(
        value=Attribute(
            value=Name(
                id="a",
                ctx=Load(),
                lineno=1,
                col_offset=1,
                end_lineno=1,
                end_col_offset=2,
            ),
            attr="b",
            ctx=Load(),
            lineno=1,
            col_offset=1,
            end_lineno=1,
            *end_col_offset=6*,
        ),
        attr="c",
        ctx=Load(),
        lineno=1,
        col_offset=1,
        end_lineno=1,
        end_col_offset=6,
    )
],

Note that the Attribute node with attr="b" has end_col_offset=6, while it should actually be 4.

New changeset d2e1098641f98594702ef29049c3c4a3f394786f by Lysandros Nikolaou in branch 'master':
bpo-39579: Fix Attribute end_col_offset to point at the current node (GH-18405)
https://github.com/python/cpython/commit/d2e1098641f98594702ef29049c3c4a3f394786f

New changeset 8b9cebce09cb6919fdb97d8e608288a503681d13 by Lysandros Nikolaou in branch '3.8':
[3.8] bpo-39579: Fix Attribute end_col_offset to point at the current node (GH-18405) (GH-18408)
https://github.com/python/cpython/commit/8b9cebce09cb6919fdb97d8e608288a503681d13

Note: this change causes a regression in kdev-python because our code was based on (and worked around) the long-standing previous behaviour; it produces broken results given the 'fixed' offset.

I shall have to write a patch with a version-check before our next stable release.

While the new behaviour is more logical, I'm not convinced that deliberate API breakage in a stable release - for something that's not a recent regression - was a good idea.

Sorry, on further inspection it was the other AST range change https://bugs.python.org/issue39474 , not this one.