MAINT: don't use f-strings in logs

There are many reason not to use f-string in logs,
 - F-strings are eager, so might be slow.
 - prevent structured logging or handler to highlight.
 - Security (untrusted input can lead to DOS on formatting,
   https://discuss.python.org/t/safer-logging-methods-for-f-strings-and-new-style-formatting/13802)
 - Performance (we can filter before formatting).

IPython/core/application.py

@@ -400,7 +400,7 b' class BaseIPythonApplication(Application):'
                     self.log.fatal("Profile %r not found."%self.profile)
                     self.exit(1)
             else:
-                self.log.debug(f"Using existing profile dir: {p.location!r}")
+                self.log.debug("Using existing profile dir: %r", p.location)
         else:
             location = self.config.ProfileDir.location
             # location is fully specified
@@ -420,7 +420,7 b' class BaseIPythonApplication(Application):'
                     self.log.fatal("Profile directory %r not found."%location)
                     self.exit(1)
             else:
-                self.log.debug(f"Using existing profile dir: {p.location!r}")
+                self.log.debug("Using existing profile dir: %r", p.location)
             # if profile_dir is specified explicitly, set profile name
             dir_name = os.path.basename(p.location)
             if dir_name.startswith('profile_'):