patch 8.1.0950: using :python sets 'pyxversion' even when not executed

Problem:    Using :python sets 'pyxversion' even when not executed.
Solution:   Check the "skip" flag. (Shane Harper, closes #3995)
diff --git a/src/testdir/test_python2.vim b/src/testdir/test_python2.vim
index d79400d..43c89c9 100644
--- a/src/testdir/test_python2.vim
+++ b/src/testdir/test_python2.vim
@@ -63,3 +63,11 @@
   py del f
   delfunc s:foo
 endfunc
+
+func Test_skipped_python_command_does_not_affect_pyxversion()
+  set pyxversion=0
+  if 0
+    python import vim
+  endif
+  call assert_equal(0, &pyxversion)  " This assertion would have failed with Vim 8.0.0251. (pyxversion was introduced in 8.0.0251.)
+endfunc