Fix the pypy3 kernel test name, and restrict pypy3 to Linux

[thomasjm]

Two failures on main from #100.

python (Linux)

kernels.pypy3 installs a kernel named python3, same as the CPython ones, so papermill couldn't
find a pypy3 kernel and the runner exited 1. testHasExpectedFields passed because it evaluates
the Nix attribute (kernels."pypy3"), which really is pypy3; only the two testKernelStdout
calls resolve the installed kernel name.

build-sample-environments (macOS, aarch64-darwin)

Enabling the pypy3 sample environment meant it stopped being an empty derivation on Darwin, and
PyPy's bootstrap interpreter doesn't build there:

implement_15.c:1486:14: error: 'ffi_prep_cif_var' is only available on macOS 10.15 or newer
  [-Werror,-Wunguarded-availability-new]

I only checked #100 on Linux. The sample environment now follows the existing optionalAttrs
pattern in sample_environments.nix, and the test is gated the same way, since the kernel it needs
can't be built on Darwin either.

Testing

pypy3 tests run green on Linux:

pypy3: has expected fields
python3 -- import platform... -> PyPy
1 runs succeeded out of 1 repeat

sample_environment_pypy3 is present in packages.x86_64-linux and absent from
packages.aarch64-darwin. Test suite compiles.