Update MANIFEST.in template.

Fixes suggested by `check-manifest`. It might be helpful to deliver
also docs and tests with releases, although they are not needed to run
the software.

MANIFEST.in

 recursive-include diceware/wordlists *.txt *.asc
-include README.rst CHANGES.rst LICENSE
+recursive-include docs *.bat *.py *.rst Makefile
+recursive-include tests *.asc *.py *.txt
+include README.rst CHANGES.rst LICENSE COPYRIGHT tox.ini