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/info.py

@@ -248,7 +248,7 @@ class Info(PagedCommand):
 
             for commit in commits:
                 split = commit.split()
-                self.text("{0:38}{1} ".format("", "-"))
+                self.text(f"{'':38}{'-'} ")
                 self.sha(split[0] + " ")
                 self.text(" ".join(split[1:]))
                 self.out.nl()