env: prevent warning from being emitted on venv creation

[layday]

After python/cpython@6811fda
the venv module produces spurious warnings for venv paths which contain
DOS-encoded parts e.g. "USER~1" in "C:\Users\USER~1".
tempfile.gettempdir() returns legacy paths like these for
user temp dirs.

MRE:

python -c "import tempfile
import venv

venv.create(tempfile.mkdtemp())"

Actual environment location may have moved due to redirects, links or junctions.
Requested location: "C:\Users\RUNNER~1\AppData\Local\Temp\tmpfoobar\Scripts\python.exe"
Actual location:    "C:\Users\runneradmin\AppData\Local\Temp\tmpfoobar\Scripts\python.exe"

Closes #413.

[layday]

@gaborbernat @henryiii could you take a look? This will unblock several other PRs.

[gaborbernat]

[uranusjr]

This is to vague (and potentially misleading because the location does not actually move; the warning is due to a misidentification of the location.

I’d say something like Call realpath so venv does not incorrectly identify equivalent path forms as different locations on Windows with a reference to the b.p.o. issue entry.

[layday]

Expanded the text and linked to the issue.

[layday]

Thanks for the feedback :)