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/branches.py

@@ -174,7 +174,7 @@ is shown, then the branch appears in all projects.
                         if _RelPath(p) not in have:
                             paths.append(_RelPath(p))
 
-                s = " %s %s" % (in_type, ", ".join(paths))
+                s = f" {in_type} {', '.join(paths)}"
                 if not i.IsSplitCurrent and (width + 7 + len(s) < 80):
                     fmt = out.current if i.IsCurrent else fmt
                     fmt(s)