Pin librt version

[ilevkivskyi]

This is to prepare for renaming native_internal -> librt.internal

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Before publishing mypy 1.19, I think we should accept any more recent versions of librt as well for mypy (use >= for version requirement), to not cause issues if a user uses an older mypy version but installs a more recent version of some third-party library that requires a more recent version of librt. Test requirements could probably continue to use the == check -- we'd just need to bump it whenever we release a new version of librt.

[ilevkivskyi]

Yeah, I already unpinned it after completing the renaming.