Apply version constraints specified with package upgrade option

[richafrank]

1. In order to get tests passing for me locally (prior to any new features/fixes), I had to update them to handle a user pipconf with a non-default index url, since the generated requirements text includes it, and the tests didn't expect it. If you'd rather I split this into a separate PR or fix it differently, let me know.
2. No longer ignore version constraints provided with the --upgrade-package option (For Upgrade specific package to a specific version fails to pick up the version constraint #649). The test I added requires a new binary wheel in the test data. I saw existing wheels there, but I'm not sure how they were generated, so I copied the contents of an existing one. Thus, the version in the wheel's contents is incorrect. Please let me know the best way to generate this test data!

Changelog-friendly one-liner: Apply version constraints specified with package upgrade option

Contributor checklist

• Provided the tests for the changes
• Requested (or received) a review from another contributor
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md afterwards).

[vphilippon]

Hello @richafrank, thanks for the contribution!

For the test wheel, the easiest way would be to write a simple setup.py like this:

from setuptools import setup

setup(
    name='small-fake-b',
    version='0.3',
)

then run python setup.py bdist_wheel --universal --dist-dir ./ to generate the wheel in the current directory.

For the change itself, LGTM 👍

[richafrank]

Thanks @vphilippon! I updated the test wheel.

[vphilippon]

Thanks @richafrank for the contribution, it's really appreciated :)

[richafrank]

👍