tox: bumped Python versions

* `envlist` looks for Python 3.8 to 3.12; these are the ones being supported [1]
* removed `py3`: it is never used, since first found wins [2]
* another bump is needed in October 2024: to take off 3.8 and 3.13 in.

```
$ tox -l
format
lint
py38
py39
py310
py311
py312
pypy3
lydevel
coverage
```

[1] https://devguide.python.org/versions/
[2] https://tox.wiki/en/stable/config.html#python-options

Signed-off-by: Ariel Otilibili <otilibil@eurecom.fr>

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = format,lint,py{36,37,38,39,310,311,312,py3,3},lydevel,coverage
+envlist = format,lint,py{38,39,310,311,312,py3},lydevel,coverage
 skip_missing_interpreters = true
 isolated_build = true
 distdir = {toxinidir}/dist