Dsub

This release includes small maintenance updates:

- `dsub`
- Quiet a warning about 'oauth2client' (which `dsub` no longer uses).
- Fix one other instance of `cache_discovery=True` raising `ImportError`.
- Add `flush` method to `_Printer`.
- Run `pytype` on `dsub`, and fix type errors
- `setup.py`: Update `dsub` dependent libraries to pick up newer versions.
- Update `tenacity` version

This release includes:

- `dsub`
- Implement `--block-external-network` to support network-sandboxed user action.
- `setup.py`: Update `dsub` dependent libraries to pick up newer versions.
- Check for python or python3 executable in local runner script. Also fix error handling for the first "write_event".
- Add zones for Seoul, Jakarta, Salt Lake City, and Las Vegas
- Documentation
- Rename references to the `master` branch to the `main` branch.

This release includes:

- `dsub`
- Update parsing of worker assigned event text to account for changes in Pipelines API.
- Hide the `--nvidia-driver-version` flag as it is now ignored by the Pipelines API (since Sept 2020).

- Documentation
- Fix link to accelerator API doc in README.
- Fix documentation that assumed `TMPDIR` is `/tmp`.
- Mention SSH from the browser in troubleshooting docs.

- Tests
- Add e2e test that confirms GPU is installed when `--accelerator-type` and `--accelerator-count` is used.
- Removed infrequently used environment variables, `CHECK_RESULTS_ONLY` and `ALLOW_DIRTY_TESTS`, from `dsub` tests.

Release `0.4.2` of `dsub` ends support Python 3.5, which [reached its "end of life" at the end of September, 2020](https://www.python.org/downloads/release/python-3510/#:~:text=Python%203.5%20will%20reach%20its,release%20of%20the%203.5%20series.).

The last version of `dsub` that supports Python 3.5 is 0.4.1.
Please use Python 3.6 or greater.

This release includes:

- Python code health
- Remove uses of `future` from `dsub`
- Remove `six` and its usages from `dsub`
- Explicitly support Python 3.6 and up.
- Feature updates
- Improvements to `dstat` output
- Use "tenacity" library instead of "retrying" library for API retries.
- Add a get_credentials function that Python clients of `dsub`, `dstat`, `ddel` can override for non-standard runtime environments.
- `google-cls-v2` provider updates:
- Use batch endpoint in `google-cls-v2` provider for job deletion (`ddel`).
- google-cls-v2: Use the batch endpoint only for `--location us-central1`.

This release includes one change to support users of Python 3.5:

- Remove a use of [f-string](https://www.python.org/dev/peps/pep-0498/)s, which was introduced in Python 3.6.

Release `0.4.0` completes the sunsetting of Python 2 support for `dsub`.
The last version of `dsub` that supports Python 2 is `0.3.10`.

This release also adds a WARNING when the `--image` flag is omitted from a call to `dsub`.
The default image is available as a getting started convenience, but for ongoing reproducible workflows, the image should be specified by the caller.
The current default is `ubuntu:14.04` which reached [End Of Life in April 2019](https://wiki.ubuntu.com/Releases). The default image will change in future releases and it is likely to be changed on a semi-regular basis, as popular base Docker linux images change.

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This release includes:
- dsub
- Update setup.py in dsub to be Python 3 only.
- Lint dsub source files as Python3 only. Fix a few lint warnings.
- Emit warning if default image is used.
- Print full path of exceptions that are retried.
- Print retry errors for socket timeout error.
- Add socket.timeout exceptions to the retry list.
- Fix markdown formatting in `dsub` README