…ox-dev#3888)

Users typing `tox -e py3.10-lint` when their envlist defined
`py310-lint` saw the command silently fall back to the base `[testenv]`
configuration, running the wrong environment without warning or error.
This became increasingly problematic with Python 3.10+ where dotted
version notation (py3.10, py3.11) is common, creating easy-to-miss typos
that produce confusing results.

Python version factors can be written with or without dots: `py3.10` vs
`py310`, `3.10` vs `310`. The validation logic treated dotted versions
as valid dynamic factors even when a normalized equivalent existed in
the config. 🔍 Since `py3.10` matched the Python version regex and `lint`
was a valid factor, `py3.10-lint` passed validation and created an
ad-hoc environment using `[testenv]` instead of the intended
`[testenv:py310-lint]`.

The fix normalizes environment names by removing dots from Python
version factors, then checks if the normalized name exists in known
environments. When found, tox errors with `py3.10-lint - did you mean
py310-lint?` instead of silently proceeding. ✨ If the normalized name
doesn't exist (e.g., `py3.15` when only `py310` and `py311` are
defined), the dotted version is allowed as an ad-hoc environment,
preserving the flexibility of tox's dynamic environment creation.

This catches typos while maintaining backward compatibility for
intentional ad-hoc environments.

Fixes tox-dev#3877

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>