sshpass: add to blacklist.

We keep rejecting this again and again.

See discussion in Homebrew/homebrew#9577, Homebrew/homebrew#12941, Homebrew/homebrew#13855.

Closes Homebrew/homebrew#18332.

Library/Homebrew/blacklist.rb

@@ -47,5 +47,9 @@ def blacklisted? name
     Installing gmock system-wide is not recommended; it should be vendored
     in your projects that use it.
     EOS
+  when 'sshpass' then <<-EOS.undent
+    We won't add sshpass because it makes it too easy for novice SSH users to
+    ruin SSH's security.
+    EOS
   end
 end