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/optwin.vim b/runtime/optwin.vim
index d759c04..82f4838 100644
--- a/runtime/optwin.vim
+++ b/runtime/optwin.vim
@@ -923,7 +923,7 @@
   call append("$", "foldmarker\tmarkers used when 'foldmethod' is \"marker\"")
   call append("$", "\t(local to window)")
   call <SID>OptionL("fmr")
-  call append("$", "foldnestmax\tmaximum fold depth for when 'foldmethod is \"indent\" or \"syntax\"")
+  call append("$", "foldnestmax\tmaximum fold depth for when 'foldmethod' is \"indent\" or \"syntax\"")
   call append("$", "\t(local to window)")
   call <SID>OptionL("fdn")
 endif
@@ -1324,6 +1324,10 @@
   call append("$", "perldll\tname of the Perl dynamic library")
   call <SID>OptionG("perldll", &perldll)
 endif
+if has('pythonx')
+  call append("$", "pyxversion\twhether to use Python 2 or 3")
+  call append("$", " \tset pyx=" . &wd)
+endif
 if exists("&pythondll")
   call append("$", "pythondll\tname of the Python 2 dynamic library")
   call <SID>OptionG("pythondll", &pythondll)