As of Pip v10+, environment variables are supported in requirements.txt
files and will be automatically interpolated. This changeset allows
Renovate to parse registry URLs conforming to that syntax properly and
follow the same approach, eg. by interpolating the environment variables
in the same way as would pip.

Note that interpolating env vars is hidden behind the trustLevel=high
flag, as described in the Renovate docs: https://github.com/renovatebot/renovate/blob/d70b8c1f2fa999be9684a8ca5da3dda8cd2392ab/docs/usage/self-hosted-configuration.md#trustlevel

Additionally, we unconditionally strip `"` characters from the same
registryUrls as those are optional in the pip spec (with no behavioural
impact) but cause lookup errors for Renovate.