Fix get_metadata return annotation and import order in misctypes

[CodyCBakerPhD]

@yarikoptic This seemed to cause some brief confusion during meeting today so split into pre-fix on its own

get_metadata() always returns a dict, never None.

The imports in RemoteReadableAsset.open() were not sorted as the pinned isort (5.12.0) of the pre-commit hook expects, so any change to dandi/misctypes.py re-sorted them.

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Claude-Session: https://claude.ai/code/session_01Qn5WBSiQgoZoL4fytF6nEr

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.40%. Comparing base (8b6a1fc) to head (35db3ae).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1950      +/-   ##
==========================================
- Coverage   78.41%   78.40%   -0.01%     
==========================================
  Files          92       92              
  Lines       14130    14130              
==========================================
- Hits        11080    11079       -1     
- Misses       3050     3051       +1     

Flag	Coverage Δ
unittests	78.40% <100.00%> (-0.01%)	⬇️

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