Include docs/requirements.txt in source distributions

MANIFEST.in

@@ -7,6 +7,7 @@ recursive-include cyipopt *.py *.pyx *.pxd
 recursive-include ipopt *.py
 recursive-include tests *.py
 recursive-include docs examples Makefile *.bat *.rst *.py
+include docs/requirements.xt
 exclude cyipopt/tests/unit/test_scipy_ipopt_from_scipy.py
 prune include*
 prune lib*