misc: Use python f-strings for string formatting

This patch has been generated by applying flynt to the
gem5 repo (ext has been excluded)

JIRA: https://gem5.atlassian.net/browse/GEM5-831

Change-Id: I0935db6223d5426b99515959bde78e374cbadb04
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/68957
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>

src/python/importer.py

@@ -50,7 +50,7 @@ class CodeImporter(object):
 
     def add_module(self, abspath, modpath, code):
         if modpath in self.modules:
-            raise AttributeError("%s already found in importer" % modpath)
+            raise AttributeError(f"{modpath} already found in importer")
 
         self.modules[modpath] = (abspath, code)