Introduce python bindings for MLIR EDSCs

This CL also introduces a set of python bindings using pybind11. The bindings
are exercised using a `test_py2andpy3.py` test suite that works for both
python 2 and 3.

`test_py3.py` on the other hand uses the more idiomatic,
python 3 only "PEP 3132 -- Extended Iterable Unpacking" to implement a rank
and type-agnostic copy with transposition.

Because python assignment is by reference, we cannot easily make the
assignment operator use the same type of sugaring as in C++; i.e. the
following:

```cpp
Stmt block = edsc::Block({
  For(ivs, zeros, shapeA, ones, {
    C[ivs] = IA[ivs] + IB[ivs]
})});
```

has no equivalent in the native Python EDSCs at this time.

However, the sugaring can be built as a simple DSL in python and is left as
future work.

PiperOrigin-RevId: 231337667