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/abandon.py

@@ -117,7 +117,7 @@ It is equivalent to "git branch -D <branchname>".
             all_projects,
             callback=_ProcessResults,
             output=Progress(
-                "Abandon %s" % (nb,), len(all_projects), quiet=opt.quiet
+                f"Abandon {nb}", len(all_projects), quiet=opt.quiet
             ),
         )
 
@@ -152,4 +152,4 @@ It is equivalent to "git branch -D <branchname>".
                             _RelPath(p) for p in success[br]
                         )
                     )
-                print("%s%s| %s\n" % (br, " " * (width - len(br)), result))
+                print(f"{br}{' ' * (width - len(br))}| {result}\n")