With the python package set moving to hash, overriding the fetcher and
reintroducing sha256 breaks eval, due to multiple hashes passed.
Also set explicit format in buildPythonApplication.
checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
