>>> from qgis.core import QGis

>>> QGis.QGIS_VERSION
u'2.18.5'
>>> QGis.QGIS_VERSION_INT
21805

>>> QGis.QGIS_RELEASE_NAME
u'Las Palmas'

try:
    from qgis.core import Qgis
except ImportError:
    from qgis.core import QGis as Qgis

from qgis.core import *
print QGis.QGIS_VERSION # will print 2.18.13