pip-compile raises an exception when setup.py has a URL in install_requires directive

[atugushev]

Environment Versions

1. OS Type: macOS
2. Python version: 3.8.0
3. pip version: 21.0.1
4. pip-tools version: 9aeeed1

Steps to replicate

1. Create setup.py

# setup.py
from setuptools import setup

setup(
    name="foo",
    version="0.1",
    install_requires=["six @ git+https://github.com/benjaminp/six#egg=six"],
)

2. Run pip-compile

Expected result

Works with pip-tools==5.5.0:

$ pip-compile
#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile
#
git+https://github.com/benjaminp/six#egg=six
    # via foo (setup.py)

Actual result

$ pip-compile
Traceback (most recent call last):
  File "/Users/albert/Projects/pip-tools/.venv/bin/pip-compile", line 33, in <module>
    sys.exit(load_entry_point('pip-tools', 'console_scripts', 'pip-compile')())
  File "/Users/albert/Projects/pip-tools/.venv/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/albert/Projects/pip-tools/.venv/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/albert/Projects/pip-tools/.venv/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/albert/Projects/pip-tools/.venv/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/albert/Projects/pip-tools/.venv/lib/python3.8/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/albert/Projects/pip-tools/piptools/scripts/compile.py", line 361, in cli
    [
  File "/Users/albert/Projects/pip-tools/piptools/scripts/compile.py", line 362, in <listcomp>
    install_req_from_req_string(req, comes_from=comes_from)
  File "/Users/albert/Projects/pip-tools/.venv/lib/python3.8/site-packages/pip/_internal/req/constructors.py", line 422, in install_req_from_req_string
    if (req.url and comes_from and comes_from.link and
AttributeError: 'str' object has no attribute 'link'

Additional context

• introduced in Use pep517 to load project metadata #1311
• install_req_from_req_string(comes_from=...) accepts only optional InsallRequirement
• install_req_from_line can be used instead of install_req_from_req_string

/cc @astrojuanlu