Much better solution, which is noticeably absent from other posts about this:
drush user-unblock {username}
https://drushcommands.com/drush-7x/user/user-unblock/
EDIT: this still doesn't clear the 'flood' table, unfortunately, which is dumb. As others have mentioned, to actually enable login after drush user-unblock
in the case of a user being blocked by too many login attempts, you have to get into the database and truncate (dump all data from) the 'flood' table.
As others have mentioned when asking this question, this seems problematic in a production environment.
As usual, someone has already made a module that adds drush commands to clear out the flood table: https://www.drupal.org/project/flood_unblock