[3.13] gh-121610: pyrepl - handle extending blocks when multi-statement blocks are pasted (GH-121757) (GH-121825)

console.compile with the "single" param throws an exception when
there are multiple statements, never allowing to adding newlines
to a pasted code block (gh-121610)

This adds a few extra checks to allow extending when in an indented
block, and tests for a few examples.

(cherry picked from commit 7d111dac16)

Co-authored-by: saucoide <32314353+saucoide@users.noreply.github.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/_pyrepl/simple_interact.py

@@ -27,6 +27,7 @@ from __future__ import annotations
 
 import _sitebuiltins
 import linecache
+import functools
 import sys
 import code
 
@@ -78,6 +79,25 @@ REPL_COMMANDS = {
 }
 
 
+def _more_lines(console: code.InteractiveConsole, unicodetext: str) -> bool:
+    # ooh, look at the hack:
+    src = _strip_final_indent(unicodetext)
+    try:
+        code = console.compile(src, "<stdin>", "single")
+    except (OverflowError, SyntaxError, ValueError):
+        lines = src.splitlines(keepends=True)
+        if len(lines) == 1:
+            return False
+
+        last_line = lines[-1]
+        was_indented = last_line.startswith((" ", "\t"))
+        not_empty = last_line.strip() != ""
+        incomplete = not last_line.endswith("\n")
+        return (was_indented or not_empty) and incomplete
+    else:
+        return code is None
+
+
 def run_multiline_interactive_console(
     console: code.InteractiveConsole,
     *,
@@ -88,6 +108,7 @@ def run_multiline_interactive_console(
     if future_flags:
         console.compile.compiler.flags |= future_flags
 
+    more_lines = functools.partial(_more_lines, console)
     input_n = 0
 
     def maybe_run_command(statement: str) -> bool:
@@ -113,16 +134,6 @@ def run_multiline_interactive_console(
 
         return False
 
-    def more_lines(unicodetext: str) -> bool:
-        # ooh, look at the hack:
-        src = _strip_final_indent(unicodetext)
-        try:
-            code = console.compile(src, "<stdin>", "single")
-        except (OverflowError, SyntaxError, ValueError):
-            return False
-        else:
-            return code is None
-
     while 1:
         try:
             try: