Rename an internal-only 'async' method kwarg for Python 3.7 compatibi…

…lity.

Fixes paramiko#1108

Cut 2.7.1

Cut 2.7.0

Cut 2.6.0

Cut 2.5.1

Cut 2.4.3

Cut 2.5.0

Cut 2.4.2

Cut 2.3.3

Cut 2.2.4