Require tox v4 and remove skipsdist/use_develop

The testenv section has `usdevelop = True` which imples skipping sdist.

In tox v4, `skipsdist = True` takes precedence over `usedevelop`and
results in the package never being installed even in `check`.

Require tox 4 version via `minversion = 4`. tox v3 will automatically
provision a environment with an upgraded version of tox, based on my
testing that raises:

   ConfigError: envdir must not equal toxinidir

Which is due to `envdir` having multiple settings and lacking an entry
for the special `.tox` environment to install tox v4 within. This set
envdir to `.tox/.pkg` which is the value used by tox v4.

Replace usage of `skipsdist`/`use_develop` which conflicts with each
other in favor of tox v4 setting `package`.

Bug: T346238
Bug: T345695
Depends-On: Ibe0492836e541d75ad84df22bedeb7c4d1bae9b2
Change-Id: I316cefef9d410c25ead3af78439170908a479c69

tox.ini

@@ -1,16 +1,19 @@
 [tox]
+minversion = 4
 envlist = check, py3-lint, black, py3-unit, integration
 
 [tox:jenkins]
 envlist = check, py{37,38,39}-{lint}, black, py{37,38,39}-{unit}, doc
 
 [testenv]
-skipsdist = True
-usedevelop = True
-
+package = editable
 # envs can be listed with:
 #
-# tox -a -v
+# tox list
+#
+# For Jenkins override you need to install in your environment:
+# https://gerrit.wikimedia.org/r/integration/tox-jenkins-override.git#egg=tox-jenkins-override
+# Then:
 # JENKINS_URL=1 tox -a -v
 #
 # envs configurations can be checked via:
@@ -33,6 +36,9 @@ description =
  doc: Build documentation using Sphinx
  venv: Dummy virtualenv to easily run commands
 envdir =
+# tox v3 provisionining would fail with:
+# ConfigError:envdir must not equal toxinidir
+ .tox: {toxworkdir}/.pkg
  py3-unit: {toxworkdir}/venv-py3
  py37-unit: {toxworkdir}/venv-py37
  py38-unit: {toxworkdir}/venv-py38
@@ -64,8 +70,7 @@ args_are_paths = False
 
 [testenv:check]
 # So we at least try sdist once
-skipsdist = False
-usedevelop = False
+package = sdist
 commands = python setup.py --version
  python setup.py check