Raise ImportError on Python 3.14 or later

[r4victor]

Fixes #3854

Currently dstack is guaranteed NOT to work on Python 3.14 due to Pydantic V1 dependency (#1844). But the installation is allowed and there are no explicit errors or warnings which leads to unexpected behavior. For example, I get segfaults on macOS:

dstack --help
[1]    67172 segmentation fault  dstack --help

The solution is to error when running/importing dstack:

dstack --help
Traceback (most recent call last):
  File "/Users/r4victor/Projects/dstack/dstack/venv314/bin/dstack", line 4, in <module>
    from dstack._internal.cli.main import main
  File "/Users/r4victor/Projects/dstack/dstack/src/dstack/__init__.py", line 4, in <module>
    raise ImportError("dstack does not support Python 3.14 or later. Please use Python 3.10–3.13.")
ImportError: dstack does not support Python 3.14 or later. Please use Python 3.10–3.13

ImportError is common in the case of Python version incompatibility so that dependent packages can handle the error if they want. CLI just prints ImportError traceback for simplicity – we could show pretty error but it would require having a separate entry point for dstack executable so that it does not import from dstack.

Specifying an upper cap like requires-python = ">=3.10, <3.14" is not an option since it would make pip backtrack to find the earliest version without the cap. uv just ignores the upper cap.