[Python] Store sequences of arbitrary ndarrays (with same type) in Tensor value type

This can be implemented using extension types, based on outcome of ARROW-1614

**Reporter**: [Wes McKinney](https://issues.apache.org/jira/browse/ARROW-5819) / @wesm
**Watchers**: [Rok Mihevc](https://issues.apache.org/jira/browse/ARROW-5819) / @rok
#### Related issues:
- [[C++] Add a Tensor logical value type with constant shape, implemented using ExtensionType](https://github.com/apache/arrow/issues/15483) (relates to)

_{**Note**: *This issue was originally created as [ARROW-5819](https://issues.apache.org/jira/browse/ARROW-5819). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}