Python: Add function to convert NumPy/Pandas dtypes to Arrow DataTypes

[asfimport]

We already provide a method to convert Arrow DataTypes to Pandas/NumPy. The reverse is yet missing.

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

_{Note: This issue was originally created as ARROW-1126. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Already implemented in https://github.com/apache/arrow/blob/master/python/pyarrow/array.pxi#L687