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/ex_cmds.h b/src/ex_cmds.h
index cb2fadc..d70ff6a 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1132,6 +1132,18 @@
 EX(CMD_py3file,		"py3file",	ex_py3file,
 			RANGE|FILE1|NEEDARG|CMDWIN,
 			ADDR_LINES),
+EX(CMD_pyx,		"pyx",		ex_pyx,
+			RANGE|EXTRA|NEEDARG|CMDWIN,
+			ADDR_LINES),
+EX(CMD_pyxdo,		"pyxdo",	ex_pyxdo,
+			RANGE|DFLALL|EXTRA|NEEDARG|CMDWIN,
+			ADDR_LINES),
+EX(CMD_pythonx,		"pythonx",	ex_pyx,
+			RANGE|EXTRA|NEEDARG|CMDWIN,
+			ADDR_LINES),
+EX(CMD_pyxfile,		"pyxfile",	ex_pyxfile,
+			RANGE|FILE1|NEEDARG|CMDWIN,
+			ADDR_LINES),
 EX(CMD_quit,		"quit",		ex_quit,
 			BANG|RANGE|COUNT|NOTADR|TRLBAR|CMDWIN,
 			ADDR_WINDOWS),