Ship requirements.txt in the sdist

setup.py reads requirements.txt to build install_requires, but setuptools does
not put it in the source distribution, so installing centml from the tarball has
always died on FileNotFoundError. Wheels carry the dependency list in their own
metadata and so were never affected, and pip prefers them — the failure only
shows up when a source build is forced.

Verified by building both distributions from this branch and installing the
sdist into a clean virtualenv: the package imports, the console script runs, and
all ten requirements resolve.

Signed-off-by: Honglin Cao <hocao@nvidia.com>

Author: V2arK

MANIFEST.in

@@ -0,0 +1 @@
+include requirements.txt