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)

src/testdir/pyxfile/py2_magic.py

@@ -0,0 +1,4 @@
+# requires python 2.x
+
+import sys
+print(sys.version)