patch 8.0.0251: not easy to select Python 2 or 3

Problem:    It is not so easy to write a script that works with both Python 2
            and Python 3, even when the Python code works with both.
Solution:   Add 'pyxversion', :pyx, etc. (Marc Weber, Ken Takata)
diff --git a/Filelist b/Filelist
index ea6f2e2..a97807e 100644
--- a/Filelist
+++ b/Filelist
@@ -122,6 +122,7 @@
 		src/testdir/pythonx/topmodule/submodule/subsubmodule/subsubsubmodule.py \
 		src/testdir/python_after/*.py \
 		src/testdir/python_before/*.py \
+		src/testdir/pyxfile/*.py \
 		src/testdir/bench*.in \
 		src/testdir/bench*.vim \
 		src/testdir/samples/*.txt \