iCloud won't let me in using a third party app
Apple send this email to all iCloud users:


  Beginning on 15 June, app-specific passwords will be required to
  access your iCloud data using third-party apps such as Microsoft
  Outlook, Mozilla Thunderbird, or other mail, contacts and calendar
  services not provided by Apple. 
  
  If you are already signed in to a third-party app using your primary
  Apple ID password, you will be signed out automatically when this
  change takes effect. You will need to generate an app-specific
  password and sign in again.


From 2017-06-15 you are required to use app-specific passwords for third party apps. 



Generate an app-specific password


Sign in to your Apple ID account page. https://appleid.apple.com
In the Security section, click Generate Password below App-Specific Passwords.
Follow the steps on your screen.


After you generate your app-specific password, enter or paste it into the password field of the app as you would normally.



Manage app-specific passwords

You can have up to 25 active app-specific passwords at any given time. If you need to, you can revoke passwords individually or all at once.


Sign in to your Apple ID account page. https://appleid.apple.com
In the Security section, click Edit.
In the App Specific Passwords section, click View History.
Click   next to a password you want to delete, or Revoke All.


After you revoke a password, the app using that password will be signed out of your account until you generate a new password and sign in again.



Any time you change or reset your primary Apple ID password, all of your app-specific passwords are revoked automatically to protect the security of your account. You'll need to generate new app-specific passwords for any apps that you want to continue using.