Improve MANIFEST.in exclusion (adamchainz#383)

Improve up on the change in adamchainz#382 to make sure that no pyc files are included from anywhere, and other files that might get added in the test directory in future would get included automatically.

Author: adamchainz

MANIFEST.in

@@ -1,4 +1,5 @@
 include HISTORY.rst README.rst
 include LICENSE
 include runtests.py tox.ini pytest.ini requirements.txt
-recursive-include tests *.py
+graft tests
+global-exclude *.py[cod] __pycache__