allow non-null post tree pre block padding

[braingram]

Description

On main if padding bytes are found between the end of the tree and the beginning of the first block they must be all null.

asdf/asdf/_block/reader.py

Lines 172 to 173 in ad470c0

	# this is the first block, allow empty bytes before block
	buff = buff.strip(b"\0")

This doesn't agree with the standard:

This empty space may be filled with any content (as long as it doesn’t contain the block_magic_token described in Blocks).

This PR updates the block parsing to allow non-null bytes as long as they don't match the block magic. Since the block header does not contain a checksum of the header bytes it's not possible to detect invalid padding (that includes the block magic). See asdf-format/asdf-standard#460

Once this is merged and asdf-format/asdf-standard#456 are merged asdf-format/asdf-standard#441 can be closed.

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]

as far as I can tell this looks good to me

might as well add a SorcerersTome class with all this reading of magic blocks