Improve migrate() in metadata.py

[candleindark]

This PR improves migrate() by removing bugs, making it more readable, adding argument validations, etc. Please see commit message for more details of the improvements.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.45%. Comparing base (6fd04fb) to head (89e2cb7).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
dandischema/metadata.py	92.68%	3 Missing ⚠️

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Flag	BASE (6fd04fb)	HEAD (89e2cb7)
unittests	528	17

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
- Coverage   97.54%   92.45%   -5.10%     
==========================================
  Files          16       16              
  Lines        1753     1775      +22     
==========================================
- Hits         1710     1641      -69     
- Misses         43      134      +91     

Flag	Coverage Δ
unittests	92.45% <93.75%> (-5.10%)	⬇️

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[yarikoptic]

note that tests are failing ATM, e.g.

________ test_migrate_value_errors[obj5-0.4.5-Current target schemas:] _________
dandischema/tests/test_metadata.py:383: in test_migrate_value_errors
    migrate(obj, to_version=target, skip_validation=True)
dandischema/metadata.py:344: in migrate
    raise ValueError(msg)
E   ValueError: Target version, '0.4.5', is not a supported target schemas, ['0.6.9']

During handling of the above exception, another exception occurred:
dandischema/tests/test_metadata.py:382: in test_migrate_value_errors
    with pytest.raises(ValueError, match=msg):
E   AssertionError: Regex pattern did not match.
E    Regex: 'Current target schemas:'
E    Input: "Target version, '0.4.5', is not a supported target schemas, ['0.6.9']"

[candleindark]

@yarikoptic Tests are updated. I think this PR is ready to be merged once you are OK with it.

[yarikoptic]

make a bit more consistent

[yarikoptic]

Suggested change

	if "schemaKey" not in instance_migrated["assetsSummary"]:
	if "schemaKey" not in instance_migrated.get("assetsSummary", {}):

[candleindark]

I replaced the line with the following instead.

        if (
            "assetsSummary" in obj_migrated
            and "schemaKey" not in obj_migrated["assetsSummary"]
        ):

[candleindark]

@yarikoptic All the issues have been handled. Please do a final check and merge.