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/runtime/doc/eval.txt b/runtime/doc/eval.txt
index ec4030e..687c605 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2239,6 +2239,7 @@
 pumvisible()			Number	whether popup menu is visible
 pyeval({expr})			any	evaluate |Python| expression
 py3eval({expr})			any	evaluate |python3| expression
+pyxeval({expr})			any	evaluate |python_x| expression
 range({expr} [, {max} [, {stride}]])
 				List	items from {expr} to {max}
 readfile({fname} [, {binary} [, {max}]])
@@ -6163,6 +6164,14 @@
 		non-string keys result in error.
 		{only available when compiled with the |+python| feature}
 
+pyxeval({expr})						*pyxeval()*
+		Evaluate Python expression {expr} and return its result
+		converted to Vim data structures.
+		Uses Python 2 or 3, see |python_x| and 'pyxversion'.
+		See also: |pyeval()|, |py3eval()|
+		{only available when compiled with the |+python| or the
+		|+python3| feature}
+
 							*E726* *E727*
 range({expr} [, {max} [, {stride}]])				*range()*
 		Returns a |List| with Numbers:
@@ -8402,6 +8411,7 @@
 profile			Compiled with |:profile| support.
 python			Compiled with Python 2.x interface. |has-python|
 python3			Compiled with Python 3.x interface. |has-python|
+pythonx			Compiled with |python_x| interface. |has-pythonx|
 qnx			QNX version of Vim.
 quickfix		Compiled with |quickfix| support.
 reltime			Compiled with |reltime()| support.