Add a digest for each get-pip.py file

[mcpate]

Relates to: https://github.com/pypa/get-pip/issues/47

This PR adds a SHA256 digest of the get-pip.py file (alongside the get-pip.py file). This seems like a useful first step towards providing mechanisms for users to validate the integrity and authenticity of the get-pip.py file.

With these changes, the output of the script now looks like this (there are now two files being output for each version in the public directories):

The SHAs can be verified in bulk as follows (the output consists of the directory, a digest for the get-pip.py file, the related digest in the *.sha246 file, and a boolean representing whether the two digests match):

>>> import hashlib
>>> import os
>>>
>>> for _path, _, _ in os.walk("./public"):
...     print(_path, end=": ")
...     hash = hashlib.sha256()
...     with open(os.path.join(_path, "get-pip.py"), "rb") as f:
...         for b_block in iter(lambda: f.read(4096), b""):
...             hash.update(b_block)
...     digest = hash.hexdigest()
...     print(digest, end=":")
...     with open (os.path.join(_path, "get-pip.py.sha256"), "r") as f:
...         file_digest = f.readlines()[0].strip()
...         print(file_digest, end=":")
...         print(digest == file_digest)
...
./public: dfe9fd5c28dc98b5ac17979a953ea550cec37ae1b47a5116007395bfacff2ab9:dfe9fd5c28dc98b5ac17979a953ea550cec37ae1b47a5116007395bfacff2ab9:True
./public/3.3: 9698fcacf9011da4f0a3cabebe679dbb1509d19b3493e8c8c9c26dd74faf5cd0:9698fcacf9011da4f0a3cabebe679dbb1509d19b3493e8c8c9c26dd74faf5cd0:True
./public/3.4: dbd5dae3d1e7f6df844d630cdf65e0f0d98e483c9997daea17c7c9d86f7b38ad:dbd5dae3d1e7f6df844d630cdf65e0f0d98e483c9997daea17c7c9d86f7b38ad:True
./public/3.5: 311afebb7cdd310eb3a3a6bb6fffef53d84493db98c7cebf4008a18d3418c8be:311afebb7cdd310eb3a3a6bb6fffef53d84493db98c7cebf4008a18d3418c8be:True
./public/3.2: bcd45dea17cff6e8b2beb383ee3cb7c25b4d30b3bac9d01fc0220d1fd3d87de9:bcd45dea17cff6e8b2beb383ee3cb7c25b4d30b3bac9d01fc0220d1fd3d87de9:True
./public/2.7: 40ee07eac6674b8d60fce2bbabc148cf0e2f1408c167683f110fd608b8d6f416:40ee07eac6674b8d60fce2bbabc148cf0e2f1408c167683f110fd608b8d6f416:True
./public/2.6: 6e1a2feaa6a90b844e36c4f8ccdc222dcebcf581a1b2f9dab09fa72e53ead0e5:6e1a2feaa6a90b844e36c4f8ccdc222dcebcf581a1b2f9dab09fa72e53ead0e5:True
./public/3.6: 0bd6aa5c457b84958cebfe1bd34aec9fa98212a65fe962dbed1195425aea58e1:0bd6aa5c457b84958cebfe1bd34aec9fa98212a65fe962dbed1195425aea58e1:True

The SHAs can also be verified directly/simply using something like sha256sum: