Make sure proc file is closed when reading

[anuraaga]

When running pytest against code that calls this library with strict warnings enabled, it fails due to proc being opened without close. In practice, I think readlines may be enough to close a file, but the ResourceWarning should still be avoided I guess.

https://github.com/connectrpc/connect-python/actions/runs/21719459903/job/62644711295?pr=118

    | Traceback (most recent call last):
    |   File "/home/runner/work/connect-python/connect-python/.venv/lib/python3.11/site-packages/find_libpython/__init__.py", line 150, in _get_proc_library
    |     lines = open(path).readlines()
    |             ^^^^^^^^^^^^^^^^^^^^^^
    | ResourceWarning: unclosed file <_io.TextIOWrapper name='/proc/2441/maps' mode='r' encoding='UTF-8'>
    | 
    | The above exception was the direct cause of the following exception:
    | 
    | Traceback (most recent call last):
    |   File "/home/runner/work/connect-python/connect-python/.venv/lib/python3.11/site-packages/_pytest/unraisableexception.py", line 67, in collect_unraisable
    |     warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))
    | pytest.PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO name='/proc/2441/maps' mode='rb' closefd=True>
    | Enable tracemalloc to get traceback where the object was allocated.
    | See https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings for more info.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.14%. Comparing base (a5a8f72) to head (5b0c60c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
+ Coverage   66.98%   67.14%   +0.15%     
==========================================
  Files           3        3              
  Lines         209      210       +1     
  Branches       51       51              
==========================================
+ Hits          140      141       +1     
  Misses         51       51              
  Partials       18       18              

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