feat: support context protocol in PythonInterpreter

[lxdlam]

Support context protocol in PythonInterpreter to enable usage such as:

with dspy.PythonInterpreter() as interp:
    result = interp(r"print('Hello, world!')")

This will automatically release the deno process and is better suited for one-shot executions.

Tests are also refactored and passed.