Update activation scripts from virtualenv

[charliermarsh]

Summary

Refreshes some of the activation scripts, and fixes some bugs in activate_this.py that were likely the rest of some erroneous copy-pasting.

Closes #3346.

Test Plan

❯ python
Python 3.12.0 (main, Feb 28 2024, 09:44:16) [Clang 15.0.0 (clang-1500.1.0.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import httpx
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'httpx'
>>> activator = '.venv/bin/activate_this.py'
>>> with open(activator) as f:
...     exec(f.read(), {'__file__': activator})
...
>>> import httpx

[charliermarsh]

Technically need to include both, though we dedup just below.

[AlexWaygood]

I know this is what virtualenv has done for a long time, and that this script is largely vendored from virtualenv, but it's very strange to me that this script monkeypatches a variable onto the sys module that wouldn't exist otherwise. I don't think the stdlib venv module does this, right?

[charliermarsh]

I haven't dug into the history deeply, but I think it's a legacy virtualenv thing, possibly from before various evolutions to the sys module came about? We don't rely on this anywhere (though pip and others have code to detect it). I'm inclined to just leave it as-is to avoid deviations, it doesn't really cost us anything.

[AlexWaygood]

Yeah. While it's still very weird to me that a script of ours would do this, I don't mind it too much given that virtualenv has been doing it since forever.