Consider improvements from Python 3.15

[Liam-DeVoe]

Python 3.15 is now in feature freeze (whatsnew), though it won't come out for another 5 months. Once it does, here are the features we should consider for Hypothesis:

• Replace our usage of bytes(bytearray(...)) with bytearray.take_bytes, where appropriate
• Replace UniqueIdentifier with sentinel.
  • This will resolve a long-standing pain point with type narrowing a UniqueIdentifier, because you cannot narrow against a specific instance of a UniqueIdentifier (but can against a sentinel).
• Investigate lazy imports (https://peps.python.org/pep-0810/)
  • I need to read the lazy imports PEP more. Is there any downside if we move all of Hypothesis to lazy imports? The upside of faster import time is clear.