Dsub

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.

Python 3 is now part of automated testing.
All unit and integration tests are now run with Python 2.7 and Python 3.7.

The `google` provider is no longer included in automated tests.

Specific changes in this release include:

- dstat
- Separate recursive input/output from input/output fields..
- Fix `YAMLLoadWarning` that was being emitted.
- dsub
- Experimental: generate 32 character UUID job ids with `--unique-job-ids` flag.
- `local` provider updates:
- Ensure runner-log.txt is written as text.
- `google-v2` provider updates:
- Add timestamps in .log files.
- Test updates:
- Remove the `google` provider from automated tests.
- Fix the way that the project ID is discovered for Python 3.
- Increase concurrency and optionally disable Python module tests.

This release includes one change:

- `google-v2` is now the default provider.

**Note**:
This is the last planned release with `google` as the default provider.
The new default provider will be `google-v2`.

This release includes:
- `google-v2` provider updates:
- `--disk-type` option added.
- `--service-account` option added.
- Fix `dstat` to find jobs submitted with user-ids containing non-alphanumeric non-hyphen characters.
- Fix `dstat` to better handle operations with no actions.