tweak stdlib imports to follow Google style guide

Google Python style guide says to import modules.
Clean up all our stdlib imports.  Leave the repo ones alone
for now as that's a much bigger shave.

Change-Id: Ida42fc2ae78b86e6b7a6cbc98f94ca04b295f8cc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/383714
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/selfupdate.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from optparse import SUPPRESS_HELP
+import optparse
 import sys
 
 from command import Command, MirrorSafeCommand
@@ -52,7 +52,7 @@ need to be performed by an end-user.
             "--repo-upgraded",
             dest="repo_upgraded",
             action="store_true",
-            help=SUPPRESS_HELP,
+            help=optparse.SUPPRESS_HELP,
         )
 
     def Execute(self, opt, args):