[3.9] bpo-45975: Use walrus operator for some idlelib while loops (GH-31083)

co-authored by Nick Drozd
cherrypicked from 51a95be1d0

Lib/idlelib/pyparse.py

@@ -179,14 +179,10 @@ def find_good_parse_start(self, is_char_in_string):
         # Peeking back worked; look forward until _synchre no longer
         # matches.
         i = pos + 1
-        while 1:
-            m = _synchre(code, i)
-            if m:
-                s, i = m.span()
-                if not is_char_in_string(s):
-                    pos = s
-            else:
-                break
+        while (m := _synchre(code, i)):
+            s, i = m.span()
+            if not is_char_in_string(s):
+                pos = s
         return pos
 
     def set_lo(self, lo):