cleanup: Update codebase to expect Python 3.6

- Bump minimum version to Python 3.6.
- Use f-strings in a lot of places.

Change-Id: I2aa70197230fcec2eff8e7c8eb754f20c08075bb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389034
Tested-by: Jason R. Coombs <jaraco@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

subcmds/prune.py

@@ -83,9 +83,7 @@ class Prune(PagedCommand):
             )
 
             if not branch.base_exists:
-                print(
-                    "(ignoring: tracking branch is gone: %s)" % (branch.base,)
-                )
+                print(f"(ignoring: tracking branch is gone: {branch.base})")
             else:
                 commits = branch.commits
                 date = branch.date