Previously top-level/python-packages.nix called spyder-kernels v0.5 for
Py2k. Now both v0.5 and v1.8 (default.nix) are in pythonPackages, as
required by cq-editor and spyder v4 respectively.
v0.5 also now comes from GitHub instead of PyPi, with checks enabled.
We should still stick with an older version for spyder 3.3.6. As soon
as 4.0.0 is released we need to upgrade this package.
This reverts commit 757fd1e27b.