Convert print statements to Python 3 style

Ran "2to3 -w -n -f print ./" and manually added imports.
Ran "^\s*print " and "\s+print " to find batch/shell scripts, comments and the like with embedded code, and updated them manually.
Also manually added imports to files, which used print as a function, but were missing the import.

The scripts still work with Python 2.
There are no intended behaviour changes.

Bug: 942522
Change-Id: Id777e4d4df4adcdfdab1b18bde89f235ef491b9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1595684
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Auto-Submit: Raul Tambre <raul@tambre.ee>
diff --git a/git_drover.py b/git_drover.py
index 05bde92..24eb077 100755
--- a/git_drover.py
+++ b/git_drover.py
@@ -4,6 +4,8 @@
 # found in the LICENSE file.
 """git drover: A tool for merging changes to release branches."""
 
+from __future__ import print_function
+
 import argparse
 import cPickle
 import functools
@@ -452,7 +454,7 @@
                          options.parent_checkout, options.dry_run,
                          options.verbose)
   except Error as e:
-    print 'Error:', e.message
+    print('Error:', e.message)
     sys.exit(128)