Merge pull request leancloud#515 from leancloud/werkzeug-two-breaking

fix: Werkzeug 2.0.0 breaking changes

Merge pull request leancloud#512 from leancloud/release-292

docs: release 2.9.2

Merge pull request leancloud#510 from leancloud/release-291

docs: release 2.9.1

Merge pull request leancloud#507 from leancloud/update-changelog-date

docs(changelog): update release date

Merge pull request leancloud#497 from leancloud/release-281

docs: release notes of 2.8.1

Merge pull request leancloud#488 from weakish/release-280

docs: 2.8.0

Merge pull request leancloud#481 from weakish/release-271

docs: release 2.7.1

Merge pull request leancloud#475 from weakish/master

docs: update api doc

Merge pull request leancloud#470 from weakish/prepare-release

docs: prepare release 2.6.1

Add `_conversationAdded` and `_conversationRemoved` hooks.