* feature: warn about unknown properties in job config

* fix: raise error instead of warning

* fix: use hasattr instead of __dict__

* fix bad merge

* fix system test that sets wrong property

Co-authored-by: Tim Swast <[email protected]>

This aligns more closely with the templates from synthtool.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

Towards #366

@tswast confirmed this is ok to merge and that broken docs-presubmit is unrelateed

…or load jobs (#482)

* fix: use project parameter if given for load jobs

* blacken client tests

* Refactor string concatenations in client tests

* Silence invalid coverage complaint

* fix: correctly convert timezone-aware datetimes

* blacken

* Remove python-dateutil test dependency

* Remove unused dst() methods

#485)

Towards #462.

With wheels for `pyarrow==3.0.0` released we can now officially support Python 3.9 - well, at least after when we add Python 3.9 checks to Kokoro.

PR checklist:
- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

🤖 I have created a release \*beep\* \*boop\* 
---
## [2.7.0](https://github.com/googleapis/python-bigquery/compare/v2.6.2...v2.7.0) (2021-01-27)


### Bug Fixes

* invalid conversion of timezone-aware datetime values to JSON ([#480](https://github.com/googleapis/python-bigquery/issues/480)) ([61b4385](https://github.com/googleapis/python-bigquery/commit/61b438523d305ce66a68fde7cb49e9abbf0a8d1d))
* reading the labels attribute on Job instances ([#471](https://github.com/googleapis/python-bigquery/issues/471)) ([80944f0](https://github.com/googleapis/python-bigquery/commit/80944f080bcc4fda870a6daf1d884de616d39ae7))
* use explicitly given project over the client's default project for load jobs ([#482](https://github.com/googleapis/python-bigquery/issues/482)) ([530e1e8](https://github.com/googleapis/python-bigquery/commit/530e1e8d8fe8939e914a78ff1b220907c1b87af7))


### Dependencies

* declare support for Python 3.9 ([#488](https://github.com/googleapis/python-bigquery/issues/488)) ([55daa7d](https://github.com/googleapis/python-bigquery/commit/55daa7da9857a8a2fb14a80a4efa3f466386a85f))
---


This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).