ci: run plugins job with python 3.9 instead of 3.7

Latest Django release dropped support for 3.7.

.github/workflows/main.yml

@@ -127,7 +127,7 @@ jobs:
             use_coverage: true
 
           - name: "plugins"
-            python: "3.7"
+            python: "3.9"
             os: ubuntu-latest
             tox_env: "plugins"
 

tox.ini

@@ -100,9 +100,9 @@ commands =
 [testenv:plugins]
 # use latest versions of all plugins, including pre-releases
 pip_pre=true
-# use latest pip and new dependency resolver (#7783)
+# use latest pip to get new dependency resolver (#7783)
 download=true
-install_command=python -m pip --use-feature=2020-resolver install {opts} {packages}
+install_command=python -m pip install {opts} {packages}
 changedir = testing/plugins_integration
 deps = -rtesting/plugins_integration/requirements.txt
 setenv =