Add Entry validation methods

[shilorigins]

Description

• Add .validate() method for all Entry subclasses
• Introduce Nestable interface for Collection and Snapshot

Motivation and Context

Validation ensures that Entrys are well formed before certain uses, such as inserting into a backend: close #11

Adding validation methods to each Entry subclass leverages polymorphism to keep validation code close to the data being validated. This also keeps the client or backend agnostic to the type of Entry being validated.

Entry.validate() uses an apischema serialization roundtrip to perform type checks. This implementation captures ValidationErrors and returns False if type checks fail, but I'd be open to leaving the exception if that's preferred. If a subclass needs specific validation tests, it can override this method; the subclass is responsible for calling Entry.validate() or otherwise performing type checks.

Nestable reduces code duplication between Collection and Snapshot, and encourages more generalized code when working with these types.

How Has This Been Tested?

Ran pytest from within the repo. Used linac_backend and new linac_snapshot fixtures to validate the original trees, a tree with an empty Collection, a tree with a cycle, and class instances that fail type checks.

Test coverage

Name                    Stmts   Miss  Cover
-------------------------------------------
__init__.py                 2      0   100%
__main__.py                 2      2     0%
_version.py                11     11     0%
backends/__init__.py        0      0   100%
backends/core.py           14      5    64%
backends/filestore.py     147     22    85%
backends/test.py           36      0   100%
bin/__init__.py             2      2     0%
bin/help.py                11     11     0%
bin/main.py                51     51     0%
bin/ui.py                  13     13     0%
client.py                  22     22     0%
errors.py                   6      0   100%
model.py                  153      6    96%
serialization.py           61     19    69%
tests/__init__.py           0      0   100%
tests/conftest.py         107      0   100%
tests/test_backend.py      71      0   100%
tests/test_model.py        79      0   100%
tests/test_window.py        5      0   100%
type_hints.py               2      0   100%
utils.py                   11      4    64%
version.py                 27     17    37%
widgets/__init__.py         0      0   100%
widgets/core.py             5      0   100%
widgets/window.py           5      0   100%
-------------------------------------------
TOTAL                     843    185    78%

Test durations

0.05s call     superscore/tests/test_window.py::test_main_window
0.04s setup    superscore/tests/test_window.py::test_main_window
0.04s call     superscore/tests/test_backend.py::test_save_entry[0]
0.02s call     superscore/tests/test_model.py::TestEntryValidation::test_fixture_validation
0.01s call     superscore/tests/test_model.py::test_serialize_snapshot_roundtrip
0.01s call     superscore/tests/test_model.py::test_serialize_collection_roundtrip
0.01s call     superscore/tests/test_backend.py::test_search_entry[0]

(47 durations < 0.005s hidden.  Use -vv to show these durations.)

Where Has This Been Documented?

This PR

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

I'm here a little late but everything here looks solid to me
I'll skip hitting approve myself because I think Robert's approval is needed following his previous review

[tangkong]

LGTM