feat(storage): mypy storage

[o-santi]

What kind of change does this PR introduce?

Fix type definitions in storage.

What is the current behavior?

mypy throws errors in both src/storage and tests.

What is the new behavior?

mypy passes for both. Also took the opportunity to improve make default message for storage.

[coveralls]

Pull Request Test Coverage Report for Build 17862390832

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 15 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.06%) to 93.795%

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Files with Coverage Reduction	New Missed Lines	%
tests/utils.py	1	54.55%
src/storage3/_sync/file_api.py	7	96.0%
src/storage3/_async/file_api.py	7	96.0%

Totals
Change from base Build 17808809082:	0.06%
Covered Lines:	8616
Relevant Lines:	9186

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💛 - Coveralls