Make sure the Sphinx theme nature is included in the sdist

--HG--
branch : distribute
extra : rebase_source : 5dfd01f1594ebc0732bd6abe2ed93c8337adbd1e

MANIFEST.in

 recursive-include setuptools *.py *.txt *.exe
 recursive-include tests *.py *.c *.pyx *.txt
-recursive-include docs *.py *.txt Makefile
+recursive-include docs *.py *.txt *.conf *.css *.css_t Makefile
 include *.py
 include *.txt
 include MANIFEST.in