py/with macro doesn't bind the return value of __enter__ method

The py/with macro implementation doesn't correctly follow Python's with statement semantics. According to the Python documentation, the value bound in a with statement should be the return value of calling __enter__() on the context manager:

manager = (EXPRESSION)
enter = type(manager).__enter__
exit = type(manager).__exit__
value = enter(manager)  # This value should be bound to TARGET
hit_except = False

try:
    TARGET = value  # TARGET gets the return value of enter(manager)
    SUITE
...

Current Implementation

The current macro (with.clj#L45-L63) calls __enter__ but discards its return value:

(py-fn/call-attr ~varname "__enter__" nil)  ; Return value is ignored
(try
  (let [retval# (do ~@body)]
    ...

Expected Behavior

The bound variable should receive the return value of __enter__(), not the context manager itself:

(let [enter-result# (py-fn/call-attr ~varname "__enter__" nil)]
  (try
    (let [~varname enter-result#  ; Rebind to __enter__'s return value
          retval# (do ~@body)]
      ...

Why Current Tests Pass

The existing test in python_test.clj#L167 uses a WithObjClass where __enter__() returns None (implicitly), and the test methods are called on the original object. This masks the bug.

Real-World Example

A common Python pattern that fails with the current implementation:

with open('file.txt', 'r') as f:
    content = f.read()  # f should be the file object returned by __enter__

With the current macro, f would be bound to the unopened file path/manager rather than the file object returned by __enter__().

Next Steps

1. Add test cases that verify this behavior (e.g., update WithObjClass.__enter__ to return a different object, or test with Python's built-in file context manager)
2. Update the macro to correctly bind the return value of __enter__()

Bắt đầu với src/libpython_clj2/python/with.clj#L45-L63 và test hiện có trong test/libpython_clj2/python_test.clj#L167. Bổ sung coverage bằng cách sử dụng một context manager có enter trả về một object khác, sau đó xác minh rằng giá trị được bind là giá trị trả về đó và behavior hiện có của with vẫn pass.