fix: pin rustpython to the same revision to fix cargo vendor (#2069)
I was trying to update ruff in nixpkgs and ran into this error when it was running `cargo vendor`
```
error: failed to sync
Caused by:
found duplicate version of package `rustpython-ast v0.2.0` vendored from two sources:
source 1: https://github.com/RustPython/RustPython.git?rev=62aa942bf506ea3d41ed0503b947b84141fdaa3c#62aa942b
source 2: https://github.com/RustPython/RustPython.git?rev=ff90fe52eea578c8ebdd9d95e078cc041a5959fa#ff90fe52
```