Fix indentation problem with _asyncify

If the input to `_asyncify` contains newlines, the strategy of indenting
and then truncating the first 8 characters won't work, since the first,
empty line doesn't get indented. Instead I've changed the format string
to accept the normal output of textwrap.indent() without modification.

IPython/core/async_helpers.py

@@ -76,7 +76,7 b' def _asyncify(code: str) -> str:'
             finally:
                 locals()
     """
-    ).format(usercode=indent(code, " " * 8)[8:])
+    ).format(usercode=indent(code, " " * 8))
     return res