I've filed a question against internal the team that manages
test accounts to ask what the recommended way to refresh these tokens are,
but have not yet gotten a response. Manually refresh the token
to unblock CI.

Drop use of 'six' wrapper library.

Drop 'u"' prefixes.

Drop support for app_engine 'classic' mode (Python 2.7-only).

Release-As: 2.0.0b1

Closes #777.

Added missing space in the log message.

Updates user guide to document using downscoping with
Credential Access Boundaries.
Regenerates references for all classes and utilities related
to this feature.

Co-authored-by: arithmetic1728 <58957152+arithmetic1728@users.noreply.github.com>

Fixes the following issues:

- Change `google.oauth2.Credentials` to `google.oauth2.credentials.Credentials`
- Replace deprecated `blob.download_as_string()` with `blob.download_as_bytes()`

We don't have a 'docfx' session in our noxfile (see #822).

Further tweak '.kokoro' configuration to get 'Kokoro docs-presubmit' running:

- Set up additional 'gfile_resource' in '.kokoro/docs/common.cfg'.
- Get 'gcloud' installed in '.kokoro/docker/docs/Dockerfile'
- Run 'docs' session with Python 3.8.

Closes #797.

* chore: split systests into their own Kokoro build

Closes #832.

* chore: docs builds no longer need systest resource

* chore: remove hacks to get 'gcloud'/'gfile_resource' installed for docs

This tests creates a temporary bucket and 2 objects in it.
A downscoped token is then created to access only one of the objects (readonly).
The test would then check:
- Read access to accessible object is successful.
- Write access to that object is unsuccessful.
- Read access to the inaccessible object is not successful.

Co-authored-by: Dan Lee <71398022+dandhlee@users.noreply.github.com>

Addresses all comments from:
#837

Co-authored-by: Tres Seaver <tseaver@palladion.com>

🤖 I have created a release \*beep\* \*boop\*
---
## [2.0.0](https://www.github.com/googleapis/google-auth-library-python/compare/v2.0.0-b1...v2.0.0) (2021-08-16)

### ⚠ BREAKING CHANGES
* drop support for Python 2.7 ([#778](https://www.github.com/googleapis/google-auth-library-python/issues/778)) ([560cf1e](https://www.github.com/googleapis/google-auth-library-python/commit/560cf1ed02a900436c5d9e0a0fb3f94b5fd98c55))

### Features

* service account is able to use a private token endpoint ([#835](https://www.github.com/googleapis/google-auth-library-python/issues/835)) ([20b817a](https://www.github.com/googleapis/google-auth-library-python/commit/20b817af8e202b0331998e5abde4e2a5aab51f9a))


### Bug Fixes

* downscoping documentation bugs ([#830](https://www.github.com/googleapis/google-auth-library-python/issues/830)) ([da8bb13](https://www.github.com/googleapis/google-auth-library-python/commit/da8bb13c1349e771ffc2e125256030495c53d956))
* Fix missing space in error message. ([#821](https://www.github.com/googleapis/google-auth-library-python/issues/821)) ([7b03988](https://www.github.com/googleapis/google-auth-library-python/commit/7b039888aeb6ec7691d91c9afce182b17f02b1a6))


### Documentation

* update user guide/references for downscoped creds ([#827](https://www.github.com/googleapis/google-auth-library-python/issues/827)) ([d1840dc](https://www.github.com/googleapis/google-auth-library-python/commit/d1840dcdcd03dfd7fdfa81d08da68402f6f8b658))
---


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