Add a `topic-venv` label

[pradyunsg]

If you want to propose a new label, state the name and the reason for its addition.

topic-venv

This would allow grouping the issues relevant to the venv module, to simplify triaging for them.

[ezio-melotti]

Ok.

[pradyunsg]

Discussed this in-person with @hugovk, @AlexWaygood and @ezio-melotti and added this. :)