Use len and str from builtins in computing dynamic prompts.

Closes: https://bugs.launchpad.net/bugs/355837

IPython/Prompts.py

@@ -128,7 +128,7 b' prompt_specials_color = {'
     r'\N': '${self.cache.prompt_count}',
     # Prompt/history count, with the actual digits replaced by dots.  Used
     # mainly in continuation prompts (prompt_in2)
-    r'\D': '${"."*len(str(self.cache.prompt_count))}',
+    r'\D': '${"."*__builtins__.len(__builtins__.str(self.cache.prompt_count))}',
     # Current working directory
     r'\w': '${os.getcwd()}',
     # Current time