pkg/54630: textproc/py-cElementTree does not build with python38

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         54630
>Category:       pkg
>Synopsis:       textproc/py-cElementTree does not build with python38
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Oct 21 06:45:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD 8.1 and others
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Building the textproc/py-cElementTree package with python38 fails
with:

  Modules/_elementtree.c:4201:10: fatal error: clinic/_elementtree.c.h: No such file or directory

This also breaks the build of devel/gobject-introspection with
PYTHON_VERSION_DEFAULT=27 due to the Python version logic discussed
in http://mail-index.netbsd.org/pkgsrc-users/2019/10/20/msg029695.html .

>How-To-Repeat:

cd pkgsrc/textproc/py-cElementTree
make PYTHON_VERSION_DEFAULT=38

>Fix: