cell objects are containers for the free variables of functions defined in a
local scope. They are located in a function's __closure__ attribute (when
it is not None). A cell is a very simple object, with a single (optional, e.g
the cell can be empty) attribute: cell_contents.

The C/Python API provides a constructor to create such objects (PyCell_New).
However no cell.__new__ method is exposed to the pure python user. Workarounds
exist, but are hacky, and involve the creation of intermediate, unused
functions.

Why would cell-creation be useful? because creating cells happens in pickle
extensions modules designed to save user-defined functions and classes
(https://github.com/cloudpipe/cloudpickle) (*).  These moudules are dependencies of
many widely-used data science frameworks (pyspark, ray, dask). Exposing a cell
constructor will simplify theses extensions code base, and alleviate their
maintenance cost.

I propose to add and expose a simple cell constructor, that accepts 0 (empty
cells) or 1 arguments. I also propose to expose the cell type in Lib/types.py
(as types.CellType)


(*): see related issues: https://bugs.python.org/issue35900