Merge pull request #7785 from nicoddemus/backport-7784

[6.0.x] Merge pull request #7784 from nicoddemus/use-new-pip-solver-7783

tox.ini

@@ -116,7 +116,11 @@ commands =
     make regen
 
 [testenv:plugins]
+# use latest versions of all plugins, including pre-releases
 pip_pre=true
+# use latest pip and new dependency resolver (#7783)
+download=true
+install_command=python -m pip --use-feature=2020-resolver install {opts} {packages}
 changedir = testing/plugins_integration
 deps =
     anyio[curio,trio]