Allow mypy to show color output

Before this change, the `TERM` environment variable was set to `dumb`,
which mypy interprets as being unable to display color. This means
that even the `FORCE_COLOR` environment variable does not force mypy to
output color.

This change sets the `TERM` environment variable to allow mypy to output
colors, making the mypy output easier to identify and interpret in CI.

See python/mypy#13817 for more information
about this problem.

Author: samueljsb

.github/workflows/tests.yml

@@ -45,7 +45,7 @@ jobs:
     - run: echo $TERM
     - run: tox run-parallel --parallel-no-spinner
       env:
-        FORCE_COLOR: "1"
+        TERM: xterm-256color
         DEFAULT_DATABASE_URL: postgres://postgres:postgres@localhost/subatomic
         OTHER_DATABASE_URL: postgres://postgres:postgres@localhost/subatomic_other