text: Title: Numeric factor in environment name wrongly causes "conflicting factors" error
  Body: https://github.com/tox-dev/tox/pull/2597 had the unintended effect of interpreting a numeric factor (e.g. `2105`) as a candidate base python, breaking previously valid environment names (e.g. `py37-foo-2105`) by raising `ValueError: conflicting factors py37, 2105 in py37-foo-2105`.
  Example GitHub workflow build: https://github.com/galaxyproject/planemo/actions/runs/3654111042/jobs/6174245903 for `tox.ini` https://github.com/galaxyproject/planemo/blob/nsoranzo-patch-1/tox.ini
  
  _Originally posted by @nsoranzo in https://github.com/tox-dev/tox/pull/2597#discussion_r1044065606_