[Python] Update minimal NumPy version to 1.16.6

[asfimport]

As part of the mitigation of numpy/numpy#17913

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

PRs and other links:

• GitHub Pull Request #8878

_{Note: This issue was originally created as ARROW-10861. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 8878
#8878