imgtool: release 1.6.0alpha3

Change-Id: I1a32938745c07c517743d2af9d0d5d2620813852
Signed-off-by: David Vincze <david.vincze@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 66e092f..10a217b 100644
--- a/scripts/imgtool/__init__.py
+++ b/scripts/imgtool/__init__.py
@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-imgtool_version = "1.6.0a2"
+imgtool_version = "1.6.0a3"