4.0.0

This is a major release for asdf that includes removal of deprecated features and changes to some defaults.

Please see the "what's new" documentation:
https://asdf.readthedocs.io/en/4.0.0/asdf/whats_new.html#whats-new-4-0-0
for a description of the included changes.

Changes include:

Feature

• Switch default ASDF standard to 1.6.0. (#1744 <https://github.com/asdf-format/asdf/pull/1744>_)
• Raise RuntimeError if a Convert subclass supports multiple tags but doesn't
  implement select_tag. (#1853 <https://github.com/asdf-format/asdf/pull/1853>_)

General

• Set memmap=False to default for asdf.open and AsdfFile.__init__.
  (#1801 <https://github.com/asdf-format/asdf/pull/1801>_)

Removal

• remove copy_arrays (replaced by memmap) (#1800 <https://github.com/asdf-format/asdf/pull/1800>_)
• Remove deprecated API. See docs for full details. (#1852 <https://github.com/asdf-format/asdf/pull/1852>_)
• Switch default convert_unknown_ndarray_subclasses to False and issue
  deprecation warning if it is enabled. (#1858 <https://github.com/asdf-format/asdf/pull/1858>_)