feat(frozen): add support for running frozen
feat(frozen): add support for running frozen

Prior to this commit, launching of notebook subprocesses was done using
`sys.executable`.  However, in frozen Python applications (e.g.,
applications packaged using [Py2Exe][py2exe]), the `sys.executable` is
likely **_not_** the Python interpreter.  In such cases, subprocess
execution will fail.

In this commit, allow Python executable path to optionally by specified
explicitly through the `PYTHONEXEPATH` environment variable (inspired by
[`platformio`][pio-pyexe]).  If not specified, fall back to the
`sys.executable` path.

[py2exe]: http://py2exe.org
[pio-pyexe]: https://github.com/platformio/platformio-core/blob/4d48c365f5b73f9b0f7d72879d1cacf0864a5bed/platformio/util.py#L687-L688

sci-bots/jupyter-helpers