Use mxdev in tox.

With standard pip, we cannot both install ourselves in editable mode *and* use a constraints file that has a pin for our package.
With mxdev, we ignore this single pin.
See plone/Products.CMFPlone#3859 (comment)

mx.ini

@@ -0,0 +1,4 @@
+[settings]
+ignores =
+    plone.app.multilingual
+main-package = -e .[test]

requirements.txt

@@ -0,0 +1 @@
+-c https://dist.plone.org/release/6.1-dev/constraints.txt

tox.ini

@@ -74,9 +74,6 @@ commands =
 
 [testenv:test]
 description = run the distribution tests
-use_develop = true
-skip_install = false
-constrain_package_deps = true
 set_env =
     ROBOT_BROWSER=headlesschrome
 
@@ -88,13 +85,17 @@ set_env =
 #  """
 ##
 deps =
+    mxdev
     zope.testrunner
     -c https://dist.plone.org/release/6.1-dev/constraints.txt
 ##
 # Specify a custom constraints file in .meta.toml:
 #  [tox]
 #  constraints_file = "https://my-server.com/constraints.txt"
 ##
+commands_pre =
+    mxdev -c mx.ini
+    pip install -rrequirements-mxdev.txt
 commands =
     zope-testrunner --all --test-path={toxinidir}/src -s plone.app.multilingual {posargs}
 extras =
@@ -111,9 +112,6 @@ extras =
 
 [testenv:coverage]
 description = get a test coverage report
-use_develop = true
-skip_install = false
-constrain_package_deps = true
 set_env =
     ROBOT_BROWSER=headlesschrome
 
@@ -128,6 +126,9 @@ deps =
     coverage
     zope.testrunner
     -c https://dist.plone.org/release/6.1-dev/constraints.txt
+commands_pre =
+    mxdev -c mx.ini
+    pip install -rrequirements-mxdev.txt
 commands =
     coverage run --branch --source plone.app.multilingual {envbindir}/zope-testrunner --quiet --all --test-path={toxinidir}/src -s plone.app.multilingual {posargs}
     coverage report -m --format markdown
@@ -154,8 +155,6 @@ commands =
 
 [testenv:circular]
 description = ensure there are no cyclic dependencies
-use_develop = true
-skip_install = false
 set_env =
 
 ##
@@ -168,9 +167,13 @@ set_env =
 allowlist_externals =
     sh
 deps =
+    mxdev
     pipdeptree
     pipforester
     -c https://dist.plone.org/release/6.1-dev/constraints.txt
+commands_pre =
+    mxdev -c mx.ini
+    pip install -rrequirements-mxdev.txt
 commands =
     # Generate the full dependency tree
     sh -c 'pipdeptree -j > forest.json'