Line continuations are not parsed correctly

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Given a file line-bug.txt with the following contents
```
requests==2.25.0 \
# comment
```
```py3
import requirements
list(requirements.parse(Path('line-bug.txt').read_text(encoding='utf8')))
```
throws an exception
```
...
~/.local/share/virtualenvs/pip-OwC7W1fl/lib/python3.8/site-packages/pkg_resources/__init__.py in parse(s)
3133 @staticmethod
3134 def parse(s):
-> 3135 req, = parse_requirements(s)
3136 return req
3137

ValueError: not enough values to unpack (expected 1, got 0)
```

But pip works fine:
```
pip install --target "`mktemp -d`" -r line-bug.txt
```

Start by reproducing the failure with the line-bug.txt contents and the requirements.parse call shown in the issue. Trace how the parser handles the continued line followed by a comment; done means this input parses without an exception, consistent with pip install -r.

Written by the indexing model from the issue text.