allow block index loading to use CSafeLoader

[braingram]

On main the block index is always loaded with SafeLoader. For files with a very large number of blocks this can take a considerable length of time.

This PR replaces the SafeLoader use with the "base loader" from asdf.versioning:

asdf/asdf/versioning.py

Line 11 in 9149ab6

_yaml_base_loader = yaml.CSafeLoader if getattr(yaml, "__with_libyaml__", None) else yaml.SafeLoader

which can be CSafeLoader.

For a file with 10000 blocks main takes ~0.5 second (more with profiling) to load the block index:

With this PR the time is about a factor of 10 less:

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