Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    :param use_pep517:  Value of the --use-pep517 option.

def parse_requirements(
    filename,  # type: str
    finder=None,  # type: Optional[PackageFinder]
    comes_from=None,  # type: Optional[str]
    options=None,  # type: Optional[optparse.Values]
    session=None,  # type: Optional[PipSession]
    constraint=False,  # type: bool
    wheel_cache=None,  # type: Optional[WheelCache]
    use_pep517=None  # type: Optional[bool]
):
    # type: (...) -> Iterator[InstallRequirement]
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    :param use_pep517:  Value of the --use-pep517 option.
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                use_pep517=use_pep517, constraint=constraint)
        for req in req_iter:
            yield req