windows tests fail

```
py: install_deps> python3 -m pip install -e . -r D:\a\python-tuf\python-tuf/requirements-test.txt
py: failed with python3 is not allowed, use allowlist_externals to allow it
```

This seems to be because we don't actually pin all build dependencies (and tox 4.0 has been released):
```
  python3 -m pip install --upgrade pip
  python3 -m pip install --upgrade tox coveralls
```

