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>

configs/learning_gem5/part3/simple_ruby.py

@@ -110,6 +110,4 @@ m5.instantiate()
 
 print("Beginning simulation!")
 exit_event = m5.simulate()
-print(
-    "Exiting @ tick {} because {}".format(m5.curTick(), exit_event.getCause())
-)
+print(f"Exiting @ tick {m5.curTick()} because {exit_event.getCause()}")