Bump version to 0.19.0

0.18.3 and release notes

update release note date

0.18.1 and release note

0.18.0 release, release notes

0.17.3 release and notes

0.17.2 and release notes

expand cloudpickle range (openai#1836)

* expand cloudpickle range

* 0.17.1 release note

fix python_requires in setup.py (openai#1827)

version -> 0.15.7 (+revert api changes)