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_python3.vim b/src/testdir/test_python3.vim
index 344034a..272ff9e 100644
--- a/src/testdir/test_python3.vim
+++ b/src/testdir/test_python3.vim
@@ -63,3 +63,11 @@
   py3 del f
   delfunc s:foo
 endfunc
+
+func Test_skipped_python3_command_does_not_affect_pyxversion()
+  set pyxversion=0
+  if 0
+    python3 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