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/test_pyx2.vim

@@ -0,0 +1,74 @@
+" Test for pyx* commands and functions with Python 2.
+
+set pyx=2
+if !has('python')
+  finish
+endif
+
+let s:py2pattern = '^2\.[0-7]\.\d\+'
+let s:py3pattern = '^3\.\d\+\.\d\+'
+
+
+func Test_has_pythonx()
+  call assert_true(has('pythonx'))
+endfunc
+
+
+func Test_pyx()
+  redir => var
+  pyx << EOF
+import sys
+print(sys.version)
+EOF
+  redir END
+  call assert_match(s:py2pattern, split(var)[0])
+endfunc
+
+
+func Test_pyxdo()
+  pyx import sys
+  enew
+  pyxdo return sys.version.split("\n")[0]
+  call assert_match(s:py2pattern, split(getline('.'))[0])
+endfunc
+
+
+func Test_pyxeval()
+  pyx import sys
+  call assert_match(s:py2pattern, split(pyxeval('sys.version'))[0])
+endfunc
+
+
+func Test_pyxfile()
+  " No special comments nor shebangs
+  redir => var
+  pyxfile pyxfile/pyx.py
+  redir END
+  call assert_match(s:py2pattern, split(var)[0])
+
+  " Python 2 special comment
+  redir => var
+  pyxfile pyxfile/py2_magic.py
+  redir END
+  call assert_match(s:py2pattern, split(var)[0])
+
+  " Python 2 shebang
+  redir => var
+  pyxfile pyxfile/py2_shebang.py
+  redir END
+  call assert_match(s:py2pattern, split(var)[0])
+
+  if has('python3')
+    " Python 3 special comment
+    redir => var
+    pyxfile pyxfile/py3_magic.py
+    redir END
+    call assert_match(s:py3pattern, split(var)[0])
+
+    " Python 3 shebang
+    redir => var
+    pyxfile pyxfile/py3_shebang.py
+    redir END
+    call assert_match(s:py3pattern, split(var)[0])
+  endif
+endfunc