python: Avoid star imports
Star Imports (from module import *) are very convenient but flake8 is
not smart enough to tell which names are imported. Replace the star
imports with specific imports.
This changes fixes these flake8 errors:
5 F403 'from .constants import *' used; unable to detect undefined names
44 F405 'MiB' may be undefined, or defined from star imports: .constants, .units
Signed-off-by: Nir Soffer <nsoffer@redhat.com>