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)

v0.16.0 release notes and version bump

0.15.6 release notes

0.15.5 release notes

release 0.15.4; bump version; release notes

bump version to 0.15.3, release notes

fix test_envs

replace release notes with 0.15.1, update Taxi env registration logic…

… to Taxi-v3