tests fail in 2032

[bmwiedemann]

• asyncpg version: asyncpg-0.27.0
• PostgreSQL version: N/A
• Python version: 3.9++
• Platform: openSUSE-Tumbleweed 20230126

While working on reproducible builds for openSUSE, I found that
our python-asyncpg package fails its tests after 2032-01-04 because tests/certs/client.cert.pem expired.

Background:
As part of my work on reproducible builds for openSUSE, I check that software still gives identical build results in the future.
The usual offset is +16 years, because that is how long I expect some software will be used in some places.
This showed up failing tests in our package build.
See https://reproducible-builds.org/ for why this matters.