Allow tests to pass after 2038

[bmwiedemann]

In openSUSE, this broke tests in python-aiosmtplib, python-anyio, python-cheroot, python-trio when running after 2038-01-01

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.

[pquentin]

Thanks, I looked at the limit we used before 2038 and it turns out it's 3000, so I switched back to that. It's OK as we require cryptography 3.1+ and the fix landed in cryptography 2.5.

[trio-bot]

Hey @bmwiedemann, it looks like that was the first time we merged one of your PRs! Thanks so much! 🎉 🎂

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