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/if_python3.c b/src/if_python3.c
index 8b4bee9..3c6ee15 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -1010,12 +1010,12 @@
 {
     char_u *script;
 
-    if (p_pyx == 0)
-	p_pyx = 3;
-
     script = script_get(eap, eap->arg);
     if (!eap->skip)
     {
+	if (p_pyx == 0)
+	    p_pyx = 3;
+
 	DoPyCommand(script == NULL ? (char *) eap->arg : (char *) script,
 		(rangeinitializer) init_range_cmd,
 		(runner) run_cmd,