Upload the Flink Python API 1.9.x to PyPI for user

I am very happy to receive some user inquiries about the use of Flink Python API (PyFlink) recently. One of the more common questions is whether it is possible to install PyFlink without using source code build. The most convenient and natural way for users is to use pip install apache-flink. We originally planned to support the use of pip install apache-flink in Flink 1.10, but the reason for this decision was that when Flink 1.9 was released at August 22, 2019[1], Flink's PyPI account system was not ready. At present, our PyPI account is available at October 09, 2019 [2](Only PMC can access), So for the convenience of users I propose:

• Publish the latest release version (1.9.2) of Flink 1.9 to PyPI.
• Update Flink 1.9 documentation to add support for pip install.

As we all know, Flink 1.9.2 was just completed released at January 31, 2020 [3]. There is still at least 1 to 2 months before the release of 1.9.3, so my proposal is completely considered from the perspective of user convenience. Although the proposed work is not large, we have not set a precedent for independent release of the Flink Python API(PyFlink) in the previous release process. I hereby initiate the current discussion and look forward to your feedback![1] https://lists.apache.org/thread.html/4a4d23c449f26b66bc58c71cc1a5c6079c79b5049c6c6744224c5f46%40%3Cdev.flink.apache.org%3E [2] https://lists.apache.org/thread.html/8273a5e8834b788d8ae552a5e177b69e04e96c0446bb90979444deee%40%3Cprivate.flink.apache.org%3E [3] https://lists.apache.org/thread.html/ra27644a4e111476b6041e8969def4322f47d5e0aae8da3ef30cd2926%40%3Cdev.flink.apache.org%3E*来自志愿者整理的flink邮件归档