The new SciPy release has a restructured `scipy.signal` namespace
and some imports in Aesara need to be refactored accordingly.
This will most probably increase the lower limit on the SciPy version.

See tracking issue #810.

Bug was introduced in b920bd29

Closes #709

This only applies to reduction `Op`s (e.g. `CAReduce`).

The rewrite would sometimes return a new graph identical to the original,
resulting in divergence.

This means that an `aesara.function`-compiled result, e.g. `func`, will expose the underlying
JITed function as `func.fn.jit_fn`.

This should retain most backward compatibility.

`TensorType` now supports full shape information.  The field
`TensorType.broadcastable` has been replaced by `TensorType.shape`, which
corresponds to the available static shape information.  `None`s are used to
encode unknown/no static shape information for a dimension.