chore: release 2.4.0

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🤖 I have created a release *beep* *boop*

2.4.0 (2021-07-12)

Features

• add always_use_jwt_access (#175) (7da3fc7)
• support self-signed JWT flow for service accounts (bfece77)

Bug Fixes

• add async client to %name_%version/init.py (bfece77)
• deps: add packaging requirement (#162) (dea0cc3)
• disable always_use_jwt_access (160a7ad)
• disable always_use_jwt_access (#181) (160a7ad)

Documentation

• omit mention of Python 2.7 in 'CONTRIBUTING.rst' (#1127) (#172) (2489e39), closes #1126

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Release is at https://github.com/googleapis/python-automl/releases/tag/v2.4.0

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