add file input error handling

[MYusufY]

This PR adds error handling for the case when the user provides a file instead of a directory.

Example:

Current version

pipreqs project/main.py:

Traceback (most recent call last):
  File "/usr/local/bin/pipreqs", line 7, in <module>
    sys.exit(main())
             ^^^^^^
  File "/usr/local/lib/python3.12/site-packages/pipreqs/pipreqs.py", line 609, in main
    init(args)
  File "/usr/local/lib/python3.12/site-packages/pipreqs/pipreqs.py", line 599, in init
    generate_requirements_file(path, imports, symbol)
  File "/usr/local/lib/python3.12/site-packages/pipreqs/pipreqs.py", line 209, in generate_requirements_file
    with _open(path, "w") as out_file:
  File "/usr/local/Cellar/python@3.12/3.12.4/Frameworks/Python.framework/Versions/3.12/lib/python3.12/contextlib.py", line 137, in __enter__
    return next(self.gen)
           ^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/pipreqs/pipreqs.py", line 91, in _open
    file = open(filename, mode)
           ^^^^^^^^^^^^^^^^^^^^
NotADirectoryError: [Errno 20] Not a directory: 'project/main.py/requirements.txt'

This fork

pipreqs project/main.py:
ERROR: The path 'project/main.py' is not a directory. Please provide a directory path.

Testing

This works properly in my tests.

Question

Is this section in contributing guideline optional?:

To run a subset of tests:

$ poetry run python -m unittest tests.test_pipreqs

Thanks!

[alan-barzilay]

its been a while since I handled this codebase, but pretty sure pipreqs can be run with an individual file as a target so this would throw an error in a legitimate use case.

Also, the project is a bit abandoned but we previously had a policy of requiring tests for any changes and new features. I dont think there are new maintainers but even if there are they may not follow this policy anymore. Do with this information what you will, sorry I cant be a bigger help