v0.28.0: Cutting version 0.28.0 of `google-cloud-core`. (#4280)

Notable Implementation Changes

• A large portion of the implementation has moved into
  google-api-core (#4022, #4041, #4057, #4067,
  #4069, #4079, #4081, #4166, #4221)

Dependencies

• Explicit depending on google-api-core and upgrading to
  grpcio >= 1.7.0 (see #4096, #4280 and
  grpc/grpc#12455)

Interface changes / additions

• Rename google.cloud.obselete module to
  obsolete (#3913, h/t to @dimaqq)

PyPI: https://pypi.org/project/google-cloud-core/0.28.0/