I hit a silly error while running unit tests under one version of python

where that installed version was built against an older ncurses that

didn't have a terminfo entry for ghostty, and the expected output wasn't

correct because the output it actually got included a preamble about how

it didn't know about xterm-ghostty.
This change adds TERM=dumb to our standard environment used when running

subprocesses. Like the existing language/ctype variables, this helps

ensure that we have a consistent environment.

Ran tests under the "broken" 3.14 install I had and saw that they now

all passed.