Fixed plone tox environment when using latest tox 4.

tox.ini

@@ -32,7 +32,7 @@ commands_pre =
 # Install Plone and explicitly the single package that uses icalendar, plus the test runner.
     pip install Plone plone.app.event[test] zope.testrunner -c https://dist.plone.org/release/6.0-dev/constraints.txt
 # Install the dev version of the package, mostly so we can safely point to the path with the tests.
-    pip install -e "git+https://github.com/plone/plone.app.event.git#egg=plone.app.event"
+    pip install -e "git+https://github.com/plone/plone.app.event.git\#egg=plone.app.event"
 # icalendar is pinned in the constraints, but we want the current dev version.
     pip install -e {toxinidir}
 commands =