Bumped version to 3.10.0a1 (#281)

diff --git a/pulsar/__about__.py b/pulsar/__about__.py
index 9be5048..3308081 100644
--- a/pulsar/__about__.py
+++ b/pulsar/__about__.py
@@ -16,4 +16,4 @@
 # specific language governing permissions and limitations
 # under the License.
 #
-__version__='3.9.0a1'
+__version__='3.10.0a1'