This patch adds asynchronous evaluation for session blocks in
Python. It also adds functionality to implement async session eval for
other languages using ob-comint.el.

To test the attached patch, add ":async" to a Python session block
with a long computation (or "time.sleep") in it. Upon evaluation, your
Emacs won't freeze to wait for the result -- instead, a placeholder
will be inserted, and replaced with the true result when it's ready.

I'll note how this is different from some related projects. ob-async
implements asynchronous evaluation for Babel, but it doesn't work with
sessions. emacs-jupyter, ein, and ob-ipython all implement
asynchronous session evaluation, but only for Jupyter kernels. Jupyter
is great for some cases, but sometimes I prefer to use the built-in
org-babel languages without jupyter.

The new functionality is mainly implemented in
`org-babel-comint-async-filter', which I've defined in ob-comint.el,
and added as a hook to `comint-output-filter-functions'.  Whenever new
output is added to the comint buffer, the filter scans for an
indicator token (this is inspired by
`org-babel-comint-with-output'). Upon encountering the token, the
filter uses a regular expression to extract a UUID or temp-file
associated with the result, then searches for the appropriate location
to add the result to.

This is my 2nd attempt at this patch [0]. I have also ported it to an
external package [1], but would like to have this functionality in Org
proper, to permit better code reuse between async and sync
implementations. The external package also includes an R
implementation that I regularly use, as well as a Ruby implementation,
but I've left these out to keep this initial patch smaller, and also I
need to confirm copyright assignment on the Ruby implementation which
was externally contributed.

[0] https://orgmode.org/list/87muj04xim.fsf@jaheira.i-did-not-set--mail-host-address--so-tickle-me/
[1] https://github.com/jackkamm/ob-session-async