Add digests to relevant files #47

From pypa/pip#5658, we could provide digests so users can verify that get-pip.py was downloaded correctly.

One use case could be during manual configuration of some server that does not have root CA certificates installed:

1. Execute curl --insecure -o get-pip.py https://bootstrap.pypa.io/get-pip.py on the server
2. Run sha256sum get-pip.py
3. Navigate to https://bootstrap.pypa.io/get-pip.py.sha256 in a web browser
4. Compare the contents of that page with the output of sha256sum