Optimize pdb with asyncio gating #156774

Summary

import pdb costs ~84 ms. The largest single contributor is asyncio at ~36 ms — imported for
exactly one call site.

Only import asyncio in pdb, if asyncio is actually being used by the program being debugged.

NOTE: initially this issue also included lazy imports optimizations, until @hugovk let me know that there was already a PR open to address that.

(PR is prepared)

Linked PRs

• gh-156774: Speed up pdb startup with asyncio guard #156775