Dsub

This release includes:
- `dsub`
- (New) (Experimental) Implemented google-cls-v2 provider that passes all tests.
- Pin all dsub dependencies to a max version
- Fix broken urls in dsub docs.
- Add dsub --summary output in wait_and_retry loop.
- `google-v2` provider updates:
- Enable a shared PID namespace when `--ssh` is specified.
- Also retry broken pipe errors
- Setting `--preemptible 0` should not cause an error.
- Testing
- Change Travis Python3 version from 3.8 to 3.7.
- Remove sorting_util_test.

This release includes:

- `dsub`
- Add periodic status update in output (via `--summary` flag).
- Update help text to clarify that `timeout` has a default of 7 days.
- Replace apiclient with googleapiclient.
- Emit a message to make it more clear that the dsub process must continue running for retries.
- Add missing quotes in documentation in example for `--mount`.
- `google-v2` provider updates:
- Update gsutil rsync warning message to include command being run.
- Add workaround for gsutil/gcloud bug that prevents re-authentication.

This release includes:

- `dsub`
- Fix RFC3339 date parsing errors with specific values under Python3
- `google-v2` provider updates:
- Filter out warning from `google-auth`.
- Retry `ResponseNotReady` error.
- Add zones for Zurich and Osaka.
- Move "sleep before retry" messages from `INFO` in stdout to `WARNING` in stderr. This ensures that the retry messages bubble up to the stderr output recorded in the pipeline's operation.

This release includes:
- `dsub`
- Explicitly reject launching jobs if blank lines found in tasks file (instead of erroring)
- `google-v2` provider updates:
- Expose the Pipelines API operation id in dsub launch stderr output.
- Replace `oauth2lib` with `google-auth`
- Update cloud sdk image to a GCR hosted one, `gcr.io/google.com/cloudsdktool/cloud-sdk:264.0.0-slim`. This version was specifically chosen as the next version updates `gsutil` from 4.42 to 4.43. 4.43 includes undesired changes to `gsutil cp`. See [gsutil's changelogs](https://github.com/GoogleCloudPlatform/gsutil/blob/master/CHANGES.md) and the [regression](https://github.com/GoogleCloudPlatform/gsutil/pull/905) for details.
- Test updates:
- Use fake time in `retrying_test`.

This release includes:
- `dsub`
- Fix cases where stderr was redirected to stdout instead of vice-versa.
- `google-v2` provider updates:
- Retry support for starting with preemptible and falling back to non-preemptible VMs
- Add a delay between gsutil cp retries
- Documentation updates:
- Remove note from README about experimental Python3 support.

This release includes:
- `dsub`
- Py3 compliance updates.
- Fix UnicodeEncodeError when task error message contains non-ascii characters
- Fix datetime.datetime.max construction used in wait loop to be an offset-aware datetime.
- Silence retrying messages during the wait loop.
- `google-v2` provider updates:
- Fix failing final_logging actions.
- Support stackdriver monitoring
- Documentation updates:
- Made it clear that multiple `--input`, `--output`, `--input-recursive`, and `--output-recursive` parameters may be used.
- Fix 'lookup' typo.