Update version to 0.0.21 in __init__.py (#16)

Author: NFUChen

py_spring_core/__init__.py

@@ -22,7 +22,7 @@
 from py_spring_core.event.application_event_publisher import ApplicationEventPublisher
 from py_spring_core.event.commons import ApplicationEvent
 
-__version__ = "0.0.20"
+__version__ = "0.0.21"
 
 __all__ = [
     "PySpringApplication",