Fix a leak when the last accessed module in multidict init is not released

[asvetlov]

Previously, the last accessed module (multidict._multidict_base actually) was not released and a reference to it was leaked.
It was not a big problem generally if multidict was not reloaded using Python code.
Anyway, cleaner code is better.

[codspeed-hq]

CodSpeed Performance Report

Merging #1061 will not alter performance

_{Comparing refactor (ed68966) with master (14cf766)}

Summary

✅ 38 untouched benchmarks