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/m5/objects/__init__.py

@@ -26,4 +26,4 @@
 
 for module in __spec__.loader_state:
     if module.startswith("m5.objects."):
-        exec("from %s import *" % module)
+        exec(f"from {module} import *")