* jupyter/kernels.scm (<kernel>): Use 'define-immutable-record-type'.
Add 'set-kernel-name' and 'set-kernel-pid'.
(find-kernel-by-name): Rename to...
(find-kernel-specs-file): ... this.
(find-kernel-specs): New procedure.
(<connection>): New record type.
(new-connection-file, kernel-arguments, exec-kernel): Remove.
(allocate-connection, spawn-kernel): New procedure.
(run-kernel): Rewrite to take a <kernel-spec> and to use
'allocate-connection'.
* tests/kernels.scm (%python3-specs): New variable.
("run-kernel python3"): Adjust accordingly.
* guix-jupyter-container.scm (reply-execute-request): Likewise.