Made some additions to the snippets

[tennyson-mccalla]

Description of the Change

Adds snippets for python 3 env header and set comprehension.

Alternate Designs

No other designs for these were considered. These were modeled after the python env header and the list comprehension that already exist as snippets.

Benefits

This minor improvement makes the snippet collection more complete and like all snippets provides easy access to perhaps frequently typed things with a few keystrokes.

Possible Drawbacks

A possible drawback is that with the "env3" snippet one may write "env" and accidentally scroll down to env3 with a mouse or keyboard. This seems unlikely and not a huge problem even if it does happen.

Applicable Issues

Fixes some minor annoyances.

Screenshots:

[winstliu]

👍