Add file

[FlaveFlav20]

I created a file-like type for PyO3. Since file is a fundamental type in Python, I believe it should be supported by this library.

Example:

use pyo3::types::pyo3file::Pyo3File;

#[pyfunction]
fn my_custom_file(mut file: Pyo3File) -> PyResult<String> {
    let mut contents = String::new();

    file.getfile().read_to_string(&mut contents)?;

    Ok(contents)
}

I created a pyo3 type because the Python file uses three parameters that are not directly available in Rust::

• name
• opening mode
• encoding (we can't know the file encoding with rust file type)

I opened this pull request to check if this approach is acceptable.