The svntest framework is Python3 since many years. Update the readme to reflect

this.

There is no code change in this commit, only documentation. It reflects my best
knowledge at the time of writing, with no other feedback on dev@ (see [1]).

* tools/dist/README.backport
  As above

[1] https://lists.apache.org/thread/bm3sfrnolqjrd5mhy7rq8lp443kqylkm


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1932433 13f79535-47bb-0310-9956-ffa450edef68

Author: d-sahlberg

tools/dist/README.backport

@@ -60,13 +60,11 @@ backport_tests_py.py:
 
 backport_tests.py:
     Common part of backport_tests_pl.py and backport_tests_py.py.  Uses the
-    svntest framework (../../subversion/tests/cmdline/svntest/), which is
-    written in Python 2.
+    svntest framework (../../subversion/tests/cmdline/svntest/).
 
-    Note that backport_tests.py and backport/*.py are written in different
-    languages, so they never 'import' each other.  backport_tests.py invokes
-    detect-conflicting-backports.py, merge-approved-backports.py, and
-    backport.pl in the same manner: through subprocess.check_call().
+    For commonality backport_tests.py invokes detect-conflicting-backports.py,
+    merge-approved-backports.py, and backport.pl in the same manner:
+    through subprocess.check_call().
 
 backport_tests_data/backport*.dump:
     Expected output files for backport_tests.py; see the BackportTest