chore: Change MPL branch, remove PYTHONPATH workarounds

[lucasmcdonald3]

Issue #, if available:

Description of changes:

• Use the "V2" MPL built on Dafny 4.7
• Remove Pythonpath hacks
• Swallow any expected exceptions in test vectors (cleaner test logs)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Check any applicable:

• [ ] Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.