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/ex_docmd.c

@@ -288,6 +288,11 @@ static void	ex_popup(exarg_T *eap);
 # define ex_py3do		ex_ni
 # define ex_py3file		ex_ni
 #endif
+#if !defined(FEAT_PYTHON) && !defined(FEAT_PYTHON3)
+# define ex_pyx			ex_script_ni
+# define ex_pyxdo		ex_ni
+# define ex_pyxfile		ex_ni
+#endif
 #ifndef FEAT_TCL
 # define ex_tcl			ex_script_ni
 # define ex_tcldo		ex_ni