Colorize mypy output on CI for easier reading

See:

- python/mypy#13815
- python/mypy#13817

This seems to be working in in all jobs *except* for the Python 3.7
and 3.8 jobs on macOS.

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -89,8 +89,11 @@ jobs:
     - name: Check types with mypy
       run: |
         mypy --python-version=${{ matrix.python-version }} -p git
-      # With new versions of mypy new issues might arise. This is a problem if there is nobody able to fix them,
-      # so we have to ignore errors until that changes.
+      env:
+        MYPY_FORCE_COLOR: "1"
+        TERM: "xterm-256color"  # For color: https://github.com/python/mypy/issues/13817
+      # With new versions of mypy new issues might arise. This is a problem if there is
+      # nobody able to fix them, so we have to ignore errors until that changes.
       continue-on-error: true
 
     - name: Test with pytest