Restrict pip’s --use-feature=2020-resolver to the extra-deps environment

tox.ini

@@ -29,8 +29,6 @@ passenv =
 download = true 
 commands =
     py.test --cov=scrapy --cov-report= {posargs:--durations=10 docs scrapy tests}
-install_command =
-    pip install --use-feature=2020-resolver {opts} {packages}
 
 [testenv:typing]
 basepython = python3
@@ -111,6 +109,8 @@ deps =
     {[testenv]deps}
     reppy
     robotexclusionrulesparser
+install_command =
+    pip install --use-feature=2020-resolver {opts} {packages}
 
 [testenv:asyncio]
 commands =