buildman: 'Thread' object has no attribute 'isAlive'

The isAlive() method was deprecated in Python 3.8 and has been removed in
Python 3.9. See https://bugs.python.org/issue37804

. Use is_alive() instead.

Since Python 2.6 is_alive() has been a synonym for isAlive(). So there
should be no problems for users using elder Python 3 versions.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>