Support strict flags in activate script

[jfhbrook]

I was using bin/activate inside a script, and noticed that it has issues with set -euo pipefail:

# ./scripts/test.sh

#!/usr/bin/env bash

set -euo pipefail

. ./.vert/bin/activate

# running it

$ bash ./scripts/test.sh 
./.vert/bin/activate: line 11: _OLD_VERT_PATH: unbound variable

It's not a huge deal - I recognize that set -euo pipefail is an incantation, and I can run set +eu in my scripts - but I don't think it would be TOO hard to test/support in those modes - in this case, writing things like "${_OLD_VERT_PATH:-}" would fix the issue. In fact, to my knowledge, venv in Python supports it, and we could probably snag an updated activate script from there.