Select Jupyter kernel for nbsphinx.

conf.py

if 'NENGO_NBSPHINX_KERNEL' in os.environ:
    nbsphinx_kernel_name = os.environ['NENGO_NBSPHINX_KERNEL']
else:
    from jupyter_client.kernelspecapp import KernelSpecManager
    kernels = sorted(KernelSpecManager().find_kernel_specs())
    print("Available Jupyter kernels:")
    for i, kernel in enumerate(kernels):
        print(" {}. {}".format(i + 1, kernel))
    nbsphinx_kernel_name = None
    while nbsphinx_kernel_name not in kernels:
        nbsphinx_kernel_name = input(
            "Select kernel for executing Jupyter notebooks: ")
        try:
            nbsphinx_kernel_name = kernels[int(nbsphinx_kernel_name) - 1]
        except (IndexError, ValueError):
            pass
    print("To make this choice permanent add the following to your shell "
          "startup file (e.g., .bashrc):")
print("export NENGO_NBSPHINX_KERNEL={!r}".format(nbsphinx_kernel_name))