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/help.py

@@ -150,7 +150,7 @@ Displays detailed usage information about a command.
     def _PrintAllCommandHelp(self):
         for name in sorted(all_commands):
             cmd = all_commands[name](manifest=self.manifest)
-            self._PrintCommandHelp(cmd, header_prefix="[%s] " % (name,))
+            self._PrintCommandHelp(cmd, header_prefix=f"[{name}] ")
 
     def _Options(self, p):
         p.add_option(