inspect imports ast but it's used only for creating signature.

But asyncio uses inspect module for unwrap(), isgenerator(), etc...
And ast module is relatively heavy for import.

This is output of python -Ximporttime -c 'import asyncio':

import time:       147 |        147 |     asyncio.constants
import time:      1456 |       1456 |           _ast
import time:       363 |       1818 |         ast
...
import time:      1890 |       5152 |       inspect
...
import time:       303 |       5984 |     asyncio.coroutines
...
import time:       731 |      37941 |   asyncio.base_events
...
import time:       375 |      41383 | asyncio

New changeset 37420deb80dcf0fc41a728838b0340b93ca01d90 by INADA Naoki in branch 'master':
bpo-32678: inspect: Import ast lazily (GH-5344)
https://github.com/python/cpython/commit/37420deb80dcf0fc41a728838b0340b93ca01d90