Update gpg warning to tell users to install python3 libs

Update gpg warning to tell users to install python3 libs now that
we're using python3

dropbox.in

@@ -58,7 +58,7 @@ from posixpath import curdir, sep, pardir, join, abspath, commonprefix
 INFO = "Dropbox is the easiest way to share and store your files online. Want to learn more? Head to"
 LINK = "https://www.dropbox.com/"
 WARNING = "In order to use Dropbox, you must download the proprietary daemon."
-GPG_WARNING = "Note: python-gpg (python-gpgme for Ubuntu 16.10 and lower) is not installed, we will not be able to verify binary signatures."
+GPG_WARNING = "Note: python3-gpg (python3-gpgme for Ubuntu 16.10 and lower) is not installed, we will not be able to verify binary signatures."
 ERROR_CONNECTING = "Trouble connecting to Dropbox servers. Maybe your internet connection is down, or you need to set your http_proxy environment variable."
 ERROR_SIGNATURE = "Downloaded binary does not match Dropbox signature, aborting install."
 ERROR_INVALID_DROPBOX = "Could not start the Dropbox daemon. Make sure your computer meets the minimum requirements:\nhttps://www.dropbox.com/help/desktop-web/system-requirements#desktop"