Use Codecov with a token

[hugovk]

Token from https://app.codecov.io/gh/prettytable/prettytable/config/general

Added to https://github.com/prettytable/prettytable/settings/secrets/actions as CODECOV_TOKEN

---

Also fix:

❯ zizmor .github/workflows/test.yml
🌈 completed test.yml
warning[artipacked]: credential persistence through GitHub Actions artifacts
  --> .github/workflows/test.yml:21:9
   |
21 |       - uses: actions/checkout@v4
   |         ------------------------- does not set persist-credentials: false
   |

1 findings (0 unknown, 0 informational, 0 low, 1 medium, 0 high)

[codecov]

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