Update %-format call to use str.format

What’s the problem this feature will solve?

Currently in src/pip/_internal/operations/prepare.py, we use %-style string formatting.

Describe the solution you’d like

We should use the .format method for consistency with the rest of the code base.

We should also consider doing the same for the rest of the code base. Whoever takes care of this specific issue should make a general issue for that!

Posted in its original form by @pradyunsg in #6968 (comment)

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This issue is a good starting point for anyone who wants to help out with pip’s development – it’s simple and the process of fixing this should be a good introduction to pip’s development workflow.

[pfmoore]

I do wonder if there’s some way to make the format strings for logging calls to use the new-style formatting

The style parameter documented here allows this. But it’s Python 3 only. Can we stop supporting Python 2 now, please? 🙂

[atugushev]

Could be related to this issue. I see now these logs on pip install ...:

Collecting pytest==3.8.2
  Downloading pytest-3.8.2-py2.py3-none-any.whl (209 kB)
     |████████████████████████████████| {downloaded} {download_speed} {pretty_eta}