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.
Including the pytest-xdist package automatically configures numprocesses
to $NIX_BUILD_CORES, so specifying these on a package level has become
redundant.
Sebastien Iooss