profiling: add an assertion to help pytype

Pytype 2023.11.21 with Python 3.10.11 (correctly) flagged `self._fp` as possibly
not having a `getvalue()` method, likely since 6a8edf9f0a6d:

    File "/mnt/c/Users/Matt/hg/mercurial/profiling.py", line 344, in __exit__:
        No attribute 'getvalue' on BinaryIO [attribute-error]
      In Union[Any, BinaryIO, io.BytesIO]

It appears this was flagged in CI too, but the test was marked as a success
anyway, so it wasn't noticed.  We'll fix that on stable and then merge on top of
this.

mercurial/profiling.py

@@ -341,7 +341,10 class profile:
                 exception_type, exception_value, traceback
             )
             if self._output == b'blackbox':
-                val = b'Profile:\n%s' % self._fp.getvalue()
+                fp = self._fp
+                # Help pytype: blackbox output uses io.BytesIO instead of a file
+                assert isinstance(fp, util.stringio)
+                val = b'Profile:\n%s' % fp.getvalue()
                 # ui.log treats the input as a format string,
                 # so we need to escape any % signs.
                 val = val.replace(b'%', b'%%')