stop pinning nightly-2020-02-05

We're currently pinning a specific nightly version in the `rust-toolchain` file. We should go back to regular `nightly` (as long as PyO3 requires it) as soon as these issues are resolved:

https://github.com/BLAKE3-team/BLAKE3/issues/56
https://github.com/BLAKE3-team/BLAKE3/issues/57
https://github.com/rust-lang/rust/issues/68905