Fix type hint in SolovayKitaevDecomposition (backport #12627)

[mergify[bot]]

Summary

The return type hint of find_basic_approximation method changes from Gate to GateSequence in SolovayKitaevDecomposition class, as implied by self.basic_approximations.

With no remaining mentions of Gate, its corresponding import statement is removed.

Details and comments

Fixes #12559.

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This is an automatic backport of pull request #12627 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 9613470946

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.04%) to 89.943%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/synthesis/unitary_synthesis.py	2	88.2%
crates/qasm2/src/lex.rs	3	92.62%
<!--	Total:	5		-->

Totals
Change from base Build 9612307245:	0.04%
Covered Lines:	61835
Relevant Lines:	68749

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💛 - Coveralls