2.7.0

The ASDF Standard is at v1.5.0.

Changes include:

• Performance improvements when reading and validating
  ASDF files.

• Numerous bug fixes. See CHANGES.rst for details.

• Add option to asdf.open that disables schema validation on read.

• Improved warning messages. All asdf library warnings now
  subclass asdf.exceptions.AsdfWarning.

• Drop support for filling default values from subschemas
  within oneOf or anyOf combiners.

• Resolve deprecation warnings from the ASDF pytest plugin
  when used with newer versions of pytest.

• Drop support for 2.x versions of the jsonschema package.