Changes for release of Ads API v10_1. (googleads#629)

* Changes for release of Ads API v10_1.

* Corrected typos in ChangeLog

* Changes for release of Ads API v10_1.

Co-authored-by: Bob Hancock <rwh@google.com>

- Bump protobuf dependency to 3.20.0 excluding 3.18.* and 3.19.*.

- Remove duplicate google-api-core dependency.

Changes for March release:

* Fix issue preventing streaming responses from being logged.
* Expose logging utilities for external interceptor logic.
* Various example updates and improvements.

Google Ads API v10_0 release

Remove support for Google Ads API v7.

- Google Ads API v9_0 release

- Update gRPC transport logic to replace "gapic" user-agent with "gccl"
- Update examples to support v9 changes
- Add new Performance Max examples
  advanced_operations/add_performance_max_campaign.py and
  shopping_ads/add_performance_max_retail_campaign.py

- Pin protobuf to < 3.18.0

- Add examples add_bidding_data_exclusion, add_bidding_seasonality_adjustment
- Style updates for examples that use search_stream
- Fix hardcoded URLs for media in various examples

* New required `use_proto_plus` configuration parameter.

* Add support for Google Ads API v8_1.
* Remove support for Google Ads API v6.

- Bump the minimum Python version requirement to 3.7+

- Fix package name in base transport files
- Update examples to new "client.enums" syntax
- Move CSV reporting example to migration directory
- Various minor updates to examples and documentation

- Google Ads API v8_0 release.

- Deprecate Google Ads API v5_0.
- *NOTE* This will be the last release to support Python 3.6.

Only install dataclasses module when < Python 3.7