Simplify open

[braingram]

Description

Simply what happens when asdf.open is called. On main this involves:

• asdf.open an alias of asdf._asdf.open_asdf
• then calls AsdfFile._open_impl a class method that expects an instance as an argument
• then calls AsdfFile._open_asdf another class method that expects an instance

This PR simplifies the process to:

• asdf.open an alias of asdf._asdf.open_asdf
• calls asdf._io.open_asdf (which reads the comments, tree and blocks)
• returns a created instance (which still involves a bunch of private attribute assignment but that can be improved in a follow-up PR)

As part of the simplification the following private API changes are made:

• add asdf._io this may expand in future PR to encapsulate more of the "low-level" IO and is currently the new home of some code that was previously part of AsdfFile including _parse_header_line _read_comment_section etc
• removes AsdfFile._fname and instead uses AsdfFile._fd._uri (the value that was assigned in asdf.open)
• removes AsdfFile._pre_write and AsdfFile._post_write
• removes _get_yaml_content argument to asdf.open (unused)

Requires: astropy/asdf-astropy#290 EDIT: merged

The pytest-asdf plugin had to be fixed as part of this work. This originally revealed some issues with the ndarray examples (where the external block example was only partially tested and the mask example fails to round-trip). See asdf-format/asdf-standard#475 for more details. The approach taken in this PR is to update the plugin to:

• convert the example to a tagged tree
• validate the tagged tree using the schema

This avoids any need to roundtrip data or provide blocks (or external blocks) which ultimately improves decoupling between the example testing and the quirks of the python asdf library.

Tasks

• run pre-commit on your machine
• run pytest on your machine
• Does this PR add new features and / or change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • update relevant docstrings and / or docs/ page
  • for any new features, add unit tests

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: bug fix
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.general.rst: infrastructure or miscellaneous change

[zacharyburnett]

this LGTM