Correct the Development Status for PyFlink

Correct the  `Development Status` value. 

From `'Development Status :: 1 - Planning'` to `'Development Status :: 5 - Production/Stable'`.

 The `Planning status` in PyPI means tell user that the package cannot be using in production [1]. So, correct the Development Status is very important for user. 

I would like to contains this fix in 1.10.0 release. 

[1] https://pypi.org/search/?c=Development+Status+%3A%3A+1+-+Planning