Release core v0.28.0

[theacodes]

Changelog:

• Rename google.cloud.obselete to obsolete (Rename google.cloud.obselete to obsolete #3913)
• Add google.api.core.gapic_v1.config (Add google.api.core.gapic_v1.config #4022)
• Add google.api.core.helpers.grpc_helpers (Add google.api.core.helpers.grpc_helpers #4041)
• Add google.api.core.gapic_v1.method (Add google.api.core.gapic_v1.method #4057)
• Add wrap_with_paging (Add wrap_with_paging #4067)
• Add grpc_helpers.create_channel (Add grpc_helpers.create_channel #4069)
• Add DEFAULT sentinel for gapic_v1.method (Add DEFAULT sentinel for gapic_v1.method #4079)
• Avoiding grpcio==1.6.0 in deps. (Avoiding grpcio==1.6.0 in deps. #4096)
• Make grpc an extra for core. (Removing googleapis-common-protos from deps in non-core packages. #4098)
• Add google.api.core.operations_v1 (Add google.api.core.operations_v1 #4081)
• Fix test assertion in test_wrap_method_with_overriding_retry_deadline (Fix test assertion in test_wrap_method_with_overriding_retry_deadline #4131)
• Add google.api.core.helpers.general_helpers.wraps (Add google.api.core.helpers.general_helpers.wraps #4166)
• Update Docs with Python Setup Guide (Add Links to Python Setup Guide #4187)
• Move modules in google.api.core.helpers up one level, delete google.api.core.helpers. (Move modules in google.api.core.helpers up one level, delete google.a… #4196)
• Clarify that PollingFuture timeout is in seconds. (Clarify that PollingFuture timeout is in seconds. #4201)
• Add documentation for google.api.core modules (Add documentation for google.api.core modules #4203)

[theacodes]

Because of #4098, should we also prepare new releases of language, logging, pubsub, spanner, speech, trace, videointelligence, and vision? Or should we just release the world?

[lukesneeringer]

If we release anything, we must release the world.

[dhermes]

This is relevant to #4186. And yes, must release the world.

[dhermes]

@jonparrott Mind making the release notes look more similar to https://github.com/GoogleCloudPlatform/google-cloud-python/releases/tag/storage-1.3.0?