Exclude compiled Python files in source distributions (#215)

diff --git a/MANIFEST.in b/MANIFEST.in
index 401787f..2c28207 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -1,2 +1,3 @@
 include README.rst LICENSE CHANGELOG.rst
 recursive-include tests *
+global-exclude *.pyc __pycache__