Use try_imports rather for conditional imports.

We're starting to use this elsewhere in OpenStack... avoids having to
do pyflakes workaround tricks.

Change-Id: Ic767677e87f30794d49baf100e3ded7555272df6
Reviewed-on: https://review.openstack.org/16851
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins
diff --git a/tools/pip-requires b/tools/pip-requires
index 703734f..e681a0c 100644
--- a/tools/pip-requires
+++ b/tools/pip-requires
@@ -6,3 +6,4 @@
 GitPython>=0.3.2.RC1
 lockfile
 python-daemon
+extras