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/src/testdir/pyxfile/pyx.py b/src/testdir/pyxfile/pyx.py
new file mode 100644
index 0000000..261a651
--- /dev/null
+++ b/src/testdir/pyxfile/pyx.py
@@ -0,0 +1,2 @@
+import sys
+print(sys.version)