[Python] Create Cython wrappers for SparseTensor

[asfimport]

We should have cython wrappers for #2546

This is related to support for https://issues.apache.org/jira/browse/ARROW-4223 and https://issues.apache.org/jira/browse/ARROW-4224

I imagine the code would be similar to 

arrow/python/pyarrow/array.pxi

Line 748 in 5a502d2

cdef class Tensor:

Reporter: Philipp Moritz / @pcmoritz
Assignee: Rok Mihevc / @rok

PRs and other links:

• GitHub Pull Request #4446

_{Note: This issue was originally created as ARROW-4453. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 4446
#4446