Modify the set pattern to hint developers that the doInstallCheck
attribute of Python packages, specified by the doCheck argument of
buildPythonPackage and buildPythonApplication, defaults to true instead
of false.
As we continue iterating on Python infrastructure for Python 3, some
code is starting to diverge for Python 2. If we copy the current state
of mk-python-derivation.nix and freeze it for Python 2, we can iterate
on it for Python 3 with more freedom.