MYPY_FORCE_COLOR only colours output on Windows, not macOS/Ubuntu

Bug Report

When using MYPY_FORCE_COLOR (or FORCE_COLOR with master) on GitHub Actions (which is not a tty), colour only shows up for Windows, and not for Ubuntu or macOS.

To Reproduce

Test with a worklflow something like this:

name: Test

on: [push, pull_request, workflow_dispatch]

env:
  MYPY_FORCE_COLOR: 1

jobs:
  build:
    runs-on: ${{ matrix.os }}
    strategy:
      fail-fast: false
      matrix:
        python-version: [
          "3.7",
          "3.10",
        ]
        os: [
          windows-latest,
          macOS-latest,
          ubuntu-latest,
        ]

    steps:
      - uses: actions/checkout@v3

      - name: Set up Python ${{ matrix.python-version }}
        uses: actions/setup-python@v4
        with:
          python-version: ${{ matrix.python-version }}

      - name: Install dependencies
        run: |
          python -m pip install -U pip
          python -m pip install mypy

      - name: Test
        run: |
          mypy test.py

Expected Behavior

Colour output for all three operating systems.

Actual Behavior

Colour output only for Windows:

And not for Ubuntu:

Nor macOS:

For example: https://github.com/hugovk/test/actions/runs/3185917456

Your Environment

• Mypy version used: 0.982
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.7 and 3.10

Start by reproducing the provided GitHub Actions workflow with MYPY_FORCE_COLOR set across Windows, macOS, and Ubuntu, then trace the command-line color handling used by mypy. Done means forced color output appears consistently on all three operating systems, including when the command is not running in a TTY.