stestr was upgraded to 3.0.0 on Mar. 26 and it does not support py27.
Update test requirements accordingly so py27 tests will use earlier version.
Change-Id: Ia481a4609d81ad4d225aeb6ec3ab0cc3a9e18ae5
Closes-Bug: 1869777
Signed-off-by: Gerry Kopec <gerry.kopec@windriver.com>