Add new-style type annotations to structs.py

[stlehmann]

This solves #175 basically by replacing the type comments with proper type annotations.

[coveralls]

Pull Request Test Coverage Report for Build 651

• 29 of 29 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.08%) to 94.617%

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Files with Coverage Reduction	New Missed Lines	%
pyads/structs.py	4	88.33%

Totals
Change from base Build 644:	0.08%
Covered Lines:	1160
Relevant Lines:	1226

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💛 - Coveralls