bpo-38673: dont switch to ps2 if the line starts with comment or whitespace (GH-17421)

https://bugs.python.org/issue38673

Misc/NEWS.d/next/Core and Builtins/2019-12-01-00-17-44.bpo-38673.K_Tze-.rst

@@ -0,0 +1 @@
+In REPL mode, don't switch to PS2 if the line starts with comment or whitespace. Based on work by Batuhan Taşkaya.

Parser/tokenizer.c

@@ -1148,6 +1148,12 @@ tok_get(struct tok_state *tok, char **p_start, char **p_end)
             if (col == 0 && c == '\n' && tok->prompt != NULL) {
                 blankline = 0; /* Let it through */
             }
+            else if (tok->prompt != NULL && tok->lineno == 1) {
+                /* In interactive mode, if the first line contains
+                   only spaces and/or a comment, let it through. */
+                blankline = 0;
+                col = altcol = 0;
+            }
             else {
                 blankline = 1; /* Ignore completely */
             }