Fix __contains__ with unexpected types

[Dreamsorcerer]

Found this while adding inline type annotations for mypy.

[codspeed-hq]

CodSpeed Performance Report

Merging #1045 will degrade performances by 17.92%

_{Comparing Dreamsorcerer-patch-1 (0045914) with master (25618eb)}

Summary

❌ 1 regressions
✅ 37 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_cimultidict_extend_istr	108.5 µs	132.2 µs	-17.92%

[asvetlov]

Nice catch!