MANIFEST.in: Exclude third_party directory.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

MANIFEST.in

diff --git a/MANIFEST.in b/MANIFEST.in
index 712f2bc..cb0dfbe 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -3,6 +3,7 @@
 exclude tox.ini
 exclude *.swp
 prune conf
+prune third_party
 
 # Include the README
 include *.md