Return True if filename points to a Python file.

def isPythonFile(filename):
    """Return True if filename points to a Python file."""
    if filename.endswith('.py'):
        return True

    # Avoid obvious Emacs backup files
    if filename.endswith("~"):
        return False

    max_bytes = 128

    try:
        with open(filename, 'rb') as f:
            text = f.read(max_bytes)
            if not text:
                return False
    except IOError:
        return False

    first_line = text.splitlines()[0]
    return PYTHON_SHEBANG_REGEX.match(first_line)