Exception in parsing cfg file

### Description

> Please leave a brief description of the bug or feature request:

### How To Reproduce
Parse this cfg file with SetupCfgHandler.
```
[metadata]
name = importlib_resources
author = Barry Warsaw
author_email = barry@python.org
description = Read resources from Python packages
long_description = file: README.rst
url = https://github.com/python/importlib_resources
classifiers =
Development Status :: 5 - Production/Stable
Intended Audience :: Developers
License :: OSI Approved :: Apache Software License
Programming Language :: Python :: 3
Programming Language :: Python :: 3 :: Only
project_urls =
Documentation = https://importlib-resources.readthedocs.io/

[options]
packages = find_namespace:
include_package_data = true
python_requires = >=3.7
install_requires =
zipp >= 3.1.0; python_version < '3.10'

[options.packages.find]
exclude =
build*
dist*
docs*
tests*

[options.extras_require]
testing =
pytest >= 6
pytest-checkdocs >= 2.4
pytest-flake8
pytest-black >= 0.3.7; \
python_implementation != "PyPy"
pytest-cov
pytest-mypy >= 0.9.1; \
python_implementation != "PyPy"
pytest-enabler >= 1.0.1
docs =
sphinx
jaraco.packaging >= 9
rst.linker >= 1.9

[egg_info]
tag_build =
tag_date = 0
```

Got this error while parsing this file
```
Traceback (most recent call last):
File "/home/tg1999/Desktop/python-inspector/venv/lib/python3.8/site-packages/dparse2/parser.py", line 74, in setuptools_parse_requirements_backport
line += next(lines)
StopIteration
File "/home/tg1999/Desktop/python-inspector/src/_packagedcode/pypi.py", line 722, in parse
dependencies = parse_with_dparse2(
File "/home/tg1999/Desktop/python-inspector/src/_packagedcode/pypi.py", line 1268, in parse_with_dparse2
dep_file = dparse2.parse(content, file_name=file_name)
File "/home/tg1999/Desktop/python-inspector/venv/lib/python3.8/site-packages/dparse2/parser.py", line 256, in parse
return dep_file.parse()
File "/home/tg1999/Desktop/python-inspector/venv/lib/python3.8/site-packages/dparse2/dependencies.py", line 139, in parse
self.parser.parse()
File "/home/tg1999/Desktop/python-inspector/venv/lib/python3.8/site-packages/dparse2/parser.py", line 236, in parse
self._parse_content(content)
File "/home/tg1999/Desktop/python-inspector/venv/lib/python3.8/site-packages/dparse2/parser.py", line 242, in _parse_content
req = parse_requirement_line(line)
File "/home/tg1999/Desktop/python-inspector/venv/lib/python3.8/site-packages/dparse2/parser.py", line 87, in parse_requirement_line
(parsed,) = setuptools_parse_requirements_backport(line)
RuntimeError: generator raised StopIteration
```
Adding one more file for input
```
[metadata]
name = importlib_metadata
author = Jason R. Coombs
author_email = jaraco@jaraco.com
description = Read metadata from Python packages
long_description = file:README.rst
url = https://github.com/python/importlib_metadata
classifiers =
Development Status :: 5 - Production/Stable
Intended Audience :: Developers
License :: OSI Approved :: Apache Software License
Programming Language :: Python :: 3
Programming Language :: Python :: 3 :: Only

[options]
packages = find_namespace:
include_package_data = true
python_requires = >=3.7
install_requires =
zipp>=0.5
typing-extensions>=3.6.4; python_version < "3.8"

[options.packages.find]
exclude =
build*
dist*
docs*
tests*
prepare*

[options.extras_require]
testing =
pytest >= 6
pytest-checkdocs >= 2.4
pytest-flake8
pytest-black >= 0.3.7; \
python_implementation != "PyPy"
pytest-cov
pytest-mypy >= 0.9.1; \
python_implementation != "PyPy"
pytest-enabler >= 1.3

importlib_resources>=1.3; python_version < "3.9"
packaging
pyfakefs
flufl.flake8
pytest-perf >= 0.9.2
docs =
sphinx
jaraco.packaging >= 9
rst.linker >= 1.9
perf =
ipython

[options.entry_points]

[egg_info]
tag_build =
tag_date = 0
```

> For bug reports, it really helps us to know:

* What OS are you running on? Linux
* What version of scancode-toolkit was used to generate the scan file?
* What installation method was used to install/run scancode? source download