Remove unused imports in tools and commands

For some non obvious reasons pep8 check doesn't detect these unused imports.

Change-Id: Ie98af5a0fcf4fb8bcd05051de75a84755fbc0b88

tools/discover_branch_points.py

@@ -14,8 +14,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from __future__ import print_function
-
 import argparse
 import atexit
 import os.path

tools/list_weeks.py

@@ -12,8 +12,6 @@
 #  License for the specific language governing permissions and limitations
 #  under the License.
 
-from __future__ import print_function
-
 import argparse
 import datetime
 

tools/tag_history_from_git.py

@@ -20,9 +20,6 @@ Use git as the canonical source of version numbers.
 
 """
 
-from __future__ import print_function
-from __future__ import unicode_literals
-
 from openstack_releases.versionutils import canonical_version
 
 import argparse

tools/verify_branches_and_late_releases.py

@@ -15,8 +15,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from __future__ import print_function
-
 import argparse
 import atexit
 import shutil