File PACKAGING_NOTES of Package python3-requests

Requests is a tricky package.

The unbundling of urllib3 is unsupported by upstream; AFAICT, mostly because
people are being unreasonable and *third-party packages* expect the bundled
urllib3 to exist. And requests upstream wants to support that usecase.

Therefore, extra care must be taken with the unbundling.

First, upstream also recognizes that we distributors will unbundle, and
actually include a helpful shim in requests/packages/__init__.py. This is to
make sure that importing `requests.packages.urllib3` works even if it is
unbundled, and also that you will get the identical import (unlike if we
installed symlinks, in which case Python would consider `r.p.urrlib3` and
`urllib3` to be different packages. See the file __init__.py for more
information.

We want to keep the shim, so we must install requests/packages/__init__.py
even though we don't install the subdirectories.

Second, it is our job to set requirements in the egg-info properly, so that
users of virtualenvs get the right thing in certain unusual configurations.

Third, requests are at times sensitive to version of urllib3. Historically,
requests even bundled pre-release versions. This will probably not happen
again, but it does mean that we must express requirements for a certain
version of urllib3. I have taken some pains to make the build fail if the
right version of urllib3 is not present. Make sure that urllib3 is updated
first, and run `set_urllib3_version.sh` to update the definition in the spec
file.