Add a requirements.txt.

Also use it within setup.py. It doesn't seem possible to define dependencies for tox with install_requires.

MANIFEST.in

@@ -7,6 +7,7 @@ include sith.py
 include conftest.py
 include pytest.ini
 include tox.ini
+include requirements.txt
 include jedi/evaluate/compiled/fake/*.pym
 include jedi/parser/python/grammar*.txt
 recursive-include test *